Is Treantmonk right or wrong about Mounted Combat?

[diplomancer]

They can if you say "overlap" to mean "same initiative order". And there is some grounds to believe that's what he meant, specifically his many statements/clarifications that "they have their own turns on the same initiative count" and that "you must choose whether the rider or mount goes first, and you can't change that order without a house-rule."

This is a humpty-dumpty argument. "Overlap" definitely does NOT mean "same initiative order", specially if what you are actually saying is "same initiative order, but separate and sequential". That is pretty much the opposite of "overlapping". Saying "they overlap, but they are separate and sequential" is to say "they overlap, but they don't overlap"

As I've said previously, JC is infuriatingly vague many times.

[Bilbron]

But "overlap" definitely does NOT mean "same initiative order", specially if what you are actually saying is "same initiative order, but separate and sequential". That is pretty much the opposite of "overlapping". It's a humpty-dumpty argument.

As I've said previously, JC is infuriatingly vague many times.

Yes, and you can definitely criticize a man for choosing the wrong word, which is something that occurs at times. I watch my own videos and occasionally see me saying something that seemed sensible at the time, but on rewatch was ambiguous, unclear, or just straight up the wrong word and I must have had a brain fart. But it does happen, and if so, we have to tease this out, which is possible based on later clarifications, all of which have seemed to point to separate, sequential turns.

I just hit him up on Twitter to complain about this glaring issue that still remains in his game, despite his efforts, so let's see if he's got it in him to give me a straight yes or no.

[Damon_Tor]

It could be relevant whose turn begins first due to certain start of turn effects and their timing. For example, if both the mount and rider are in the area of a Cloudkill, they would both take the start-of-turn damage if they both start their shared turn at the same time. But if their turns are separate but overlapping, the mount could begin its turn and take the damage, then (assuming it survives) ride out of the cloud before the rider starts his turn, saving the rider from taking the damage.

[Bricon]

It's been stated but I literally made an account to chime in,


"The initiative of a controlled mount changes to match yours when you mount it. ... A controlled mount can move and act even on the turn that you mount it"

Basically, RAW, anytime two creatures have the same initiative count, you have to decide the turn order between them and the turns are resolved sequentially, NOT simultaneously. I guess the point of contention is whether the can move and act even on the turn that you mount it line is an exception only on that turn or an exception to all the mount's turns. And Crawford's use of "overlap" in his interview is curious.

I rule it as you have to give them separate turns in a sequential order based on this. However, I think it's way less fun to do it that way and I like TM's interpretation.

[Witty Username]

Also, separate but overlapping turns covers a loophole, if a mount could move and act on your turn and on its own non-overlapping turn it could move and act twice during the round. If It takes its turn overlapping with yours then it can move and act during your turn without allowing to move twice.

[noob]

Since that only works with a controlled mount, your fighter would both have to have specific mount training, and have an intelligence below 3. So if you train your especially dumb fighter friend, go for it.


On pg. 198 of the PHB it states;
While you're mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently. You can control a mount only if it has been trained to accept a rider.

So now a sufficiently dumped int can be weaponised?

[Tanarii]

"The initiative of a controlled mount changes to match yours when you mount it. ... A controlled mount can move and act even on the turn that you mount it"

Basically, RAW, anytime two creatures have the same initiative count, you have to decide the turn order between them and the turns are resolved sequentially, NOT simultaneously. I guess the point of contention is whether the can move and act even on the turn that you mount it line is an exception only on that turn or an exception to all the mount's turns. And Crawford's use of "overlap" in his interview is curious.

Neither. It changes to match yours. Then it can use its turn, which now matches yours, immediately after it's done.

It is poorly worded, but "the turn that you mount it" is its first immediately available turn, right after you mount it.

[Osuniev]

Neither. It changes to match yours. Then it can use its turn, which now matches yours, immediately after it's done.

It is poorly worded, but "the turn that you mount it" is its first immediately available turn, right after you mount it.

Yes, that's how I rule it too. The only exception would be Small Size Beastmasters riding their pet.

It doesn't make mounts useless, it gives you a free 60 feet of movement without Attacks of pportunity (if the mount disengages) or a 120 feet of Movement (if the mount dashes) before or after your turn, basically. At the cost of half your own movement if you want to mount/dismount. That's pretty powerful.

It does prevent you from doing hit-and-run tactics without AoO, (unless you used a Ready action and therefore forego Extra Attacks), which is good, I believe.

Mounts are still crazy useful.

"the mount can act the turn that you mount it" was, IMO, intended to clarify that you don't need to wait until next turn to gain that benefit (even if your mount already acted that round before you mounted it)

[greenstone]

Also, separate but overlapping turns covers a loophole, if a mount could move and act on your turn and on its own non-overlapping turn it could move and act twice during the round. If It takes its turn overlapping with yours then it can move and act during your turn without allowing to move twice.

My reading is that this is the situation that paragraph is covering - the ability for the mount to act twice in a round.

To illustrate:
Start of combat, character C rolls initiative 13, their horse (not carrying a rider) rolls 16.

Round 1:
On count 16 the horse has its turn.
On count 13 the character has their turn and mounts the horse. The horse's initiative changes to 13 and, as per the rulebook, now gets a turn (it's second this round).

[KorvinStarmast]

Clearly the more important question is, can I, a halfling, mount my fighter friend immediately after his turn ends to give him a second turn?

Make sure to offer the Fighter a cigarette afterwards.

Tanarii's method is what I use. It's as close to the KISS principle as the hot mess that is mounted combat comes when you do it that way.

[Witty Username]

My reading is that this is the situation that paragraph is covering - the ability for the mount to act twice in a round.

To illustrate:
Start of combat, character C rolls initiative 13, their horse (not carrying a rider) rolls 16.

Round 1:
On count 16 the horse has its turn.
On count 13 the character has their turn and mounts the horse. The horse's initiative changes to 13 and, as per the rulebook, now gets a turn (it's second this round).

Technically, I was thinking
That rider would mount, Mount would get a move and action turn and change initiative. (Mount moves once)
Then mount would move and act on it's new initiative. (mount moves again).
Unless the turn the mount gets overlaps with yours in which case it gets one.

But I think your example works too, since if its turn is not how it moves with you it would have to move twice to obey both rules.

[diplomancer]

Yes, that's how I rule it too. The only exception would be Small Size Beastmasters riding their pet.

It doesn't make mounts useless, it gives you a free 60 feet of movement without Attacks of pportunity (if the mount disengages) or a 120 feet of Movement (if the mount dashes) before or after your turn, basically. At the cost of half your own movement if you want to mount/dismount. That's pretty powerful.

It does prevent you from doing hit-and-run tactics without AoO, (unless you used a Ready action and therefore forego Extra Attacks), which is good, I believe.

Mounts are still crazy useful.

"the mount can act the turn that you mount it" was, IMO, intended to clarify that you don't need to wait until next turn to gain that benefit (even if your mount already acted that round before you mounted it)

At a cost of 400 GP for a Warhorse, mounts SHOULD be crazy useful; more useful than what this ruling allows for (which, incidentally, does not stop hit and run tactics, really, only hit and run tactics by melee attackers with more than one attack)

Make sure to offer the Fighter a cigarette afterwards.

Or some oats, if that's his thing

Tanarii's method is what I use. It's as close to the KISS principle as the hot mess that is mounted combat comes when you do it that way.

Why is it simpler than just having them work as a unit? I played an entire campaign with a Paladin from 6th to 20th level, with Mounted Combatant from 8th, and never saw anything complicated about how it works, no conflicts, no need for the DM to adjudicate any corner cases, etc.
I find the added artificiality of readying actions (which is yet another nerf to melee combatants, the bigger the nerf the more attacks they have) far less simple.

Neither. It changes to match yours. Then it can use its turn, which now matches yours, immediately after it's done.

It is poorly worded, but "the turn that you mount it" is its first immediately available turn, right after you mount it.

I will just note that this argument is "PHB is wrong, the way I rule it is the correct one; even though it's not what the book says, it is what it wanted to say"

You are of course free to rule it that way in your game. But the book says what it says, and overlapping the turns is more consistent with it than keeping them separate.

It could be relevant whose turn begins first due to certain start of turn effects and their timing. For example, if both the mount and rider are in the area of a Cloudkill, they would both take the start-of-turn damage if they both start their shared turn at the same time. But if their turns are separate but overlapping, the mount could begin its turn and take the damage, then (assuming it survives) ride out of the cloud before the rider starts his turn, saving the rider from taking the damage.

I think you've solved the riddle; the turns DO overlap but it's important to determine which one starts first because of these effects that trigger on a turn's start. It's also why you can't switch it afterwards (or it would be abusable)

[Tanarii]

I will just note that this argument is "PHB is wrong, the way I rule it is the correct one; even though it's not what the book says, it is what it wanted to say"

You are of course free to rule it that way in your game. But the book says what it says, and overlapping the turns is more consistent with it than keeping them separate.

Nope. It's "here's how I interpret what the PHB says, it's just not phrased particularly well, it could have been clearer.

[Zhorn]

Count me as another that reads it the same as Tanarii's, not concurrent turns, just matching and sequential initiatives. If mounted and controlled after their turn in a round, the mount takes the very next turn in initiative order.

Language in RAW is sloppy for controlled mounts and that line of "on the same turn you mounted it" very much seems like an oversight of language when considering how turns behave.

Being said, I don't kick up a stink on the matter if a DM I'm playing with wants to simplify it to the rider just uses the mount's movement speed on the same turn.

[MaxWilson]

Count me as another that reads it the same as Tanarii's, not concurrent turns, just matching and sequential initiatives. If mounted and controlled after their turn in a round, the mount takes the very next turn in initiative order.

Language in RAW is sloppy for controlled mounts and that line of "on the same turn you mounted it" very much seems like an oversight of language when considering how turns behave.

If we're going to view RAW as sloppy, it also makes sense to view "its initiative changes to match yours" as "its turn changes to match (overlap with) yours." This is consistent with the next sentence about being able to move even on the turn when you mount it.

[MustaKrakish]

I understand why people want it to work that way, obviously, it's logical. But D&D is a turn-based game, and there are rules for tied initiative weird wording of the rules or not, it's clear that people are trying really hard to ignore this. But, you can still work around it somehow.

Mount starts first, move, then ready a dash action for after the character uses the attack action, then mount trigger said dash action and move again, character attacks again if they have a bonus action attack or something like that.

I would only allow for a "combined turn" if I would create a totally new house rule, and then, like people already mentioned here, things might use bonus actions, etc'.

[Valmark]

I understand why people want it to work that way, obviously, it's logical. But D&D is a turn-based game, and there are rules for tied initiative weird wording of the rules or not, it's clear that people are trying really hard to ignore this. But, you can still work around it somehow.

Mount starts first, move, then ready a dash action for after the character uses the attack action, then mount trigger said dash action and move again, character attacks again if they have a bonus action attack or something like that.

I would only allow for a "combined turn" if I would create a totally new house rule, and then, like people already mentioned here, things might use bonus actions, etc'.

Controlled mounts cannot Ready actions, unfortunately.

And it's not really a matter of ignoring it- simply put, the mount cannot move and act in the same turn that you mount it without... Doing so. Sequential turns aren't the same turn.

[diplomancer]

I understand why people want it to work that way, obviously, it's logical. But D&D is a turn-based game, and there are rules for tied initiative weird wording of the rules or not, it's clear that people are trying really hard to ignore this. But, you can still work around it somehow.

Mount starts first, move, then ready a dash action for after the character uses the attack action, then mount trigger said dash action and move again, character attacks again if they have a bonus action attack or something like that.

I would only allow for a "combined turn" if I would create a totally new house rule, and then, like people already mentioned here, things might use bonus actions, etc'.

The people who are trying really hard to ignore anything are the ones who claim that "the mount can move and act on the same turn in which you mount it" does not mean what it says and, in fact, means something else like "on the turn right afterwards".

And for what? To make mounted combat more complicated, more artificial, less effective, and less fun, all in the name of an abstract, general rule, to which mounted combat in a controlled mount is quite a natural exception.

[MaxWilson]

The people who are trying really hard to ignore anything are the ones who claim that "the mount can move and act on the same turn in which you mount it" does not mean what it says and, in fact, means something else like "on the turn right afterwards"

It's not like there isn't precedent for multiple creatures acting on the same turn.

[KorvinStarmast]

At a cost of 400 GP for a Warhorse, mounts SHOULD be crazy useful; more useful than what this ruling allows for

agree.

Why is it simpler than just having them work as a unit?

Once the 'mounting turn is over, they do.
If someone is pre mounted, I have them work on the same initiative order number because it's simpler that way for me as a DM. None of my players has yet taken mounted combatant. I gave it to a few bands of gnolls, mounted in dire wolves - they had the mounted combatant feature. Certainly made taking the dire wolves down a bit more difficult.

I find the added artificiality of readying actions (which is yet another nerf to melee combatants, the bigger the nerf the more attacks they have) far less simple.

Yeah, any fighter, barb, monk, paladin of 5th level or greater gets hosed by the way extra attack is all of a sudden ignored whan an OA happens ...

I think you've solved the riddle; the turns DO overlap but it's important to determine which one starts first because of these effects that trigger on a turn's start. It's also why you can't switch it afterwards (or it would be abusable)

yes

[BigRedJedi]

Mounted Rules From PHB:

Spoiler

Show

While you’re mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently.

You can control a mount only if it has been trained to accept a rider. Domesticated horses, donkeys, and similar creatures are assumed to have such training. The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.

An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. It might flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes.

In either case, if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.

A controlled mount "moves as you direct it" implies that the mount is sharing your turn (including substituting the use of its movement in place of the rider's), rather than acting independently on its own turn, as independent mounts explicitly act, separately on their own initiative. The limits on actions that a controlled mount can take would seem to reinforce this perspective.

Yes, this does mean that if a warhorse has already moved and acted in a round, once mounted, it can move and/or take one of the 3 explicitly listed actions during the turn of the rider. (For emphasis, controlled mounts don't get to make attacks or use special abilities while mounted, so using your fighter friend as a mount will be very disappointing for him.)

Tangentially, note that a controlled mount cannot take the Ready action. Further, Readying the Dash action is a complete waste by RAW, as Dash only increases your effective speed, used while taking the Move portion of your turn, rather than being an explicit action that contains movement. Similarly, the Ready action lets you either take a normal action available on your turn or move up to your speed, in response to a predetermined trigger. Conceptually, Readying to move is similar to Readying the Dash action, but RAW makes this a waste. (One of many silly functions of the mechanics of Readying actions in 5E.)

[Thunderous Mojo]

It might be useful to examine the verbiage of "Pet" abilities that were subsequently published after the PHB, as those abilities should hopefully have clearer language that may elucidate the Mounted Combat rules as written in the PHB.

The Wildfire Spirit ability from the Circle of Wildfire states this:
In combat,the spirits share your initiative count,
but it takes its turn immediately after yours.

The Primal Companion alternative ability for Beast Master Ranger's states this: In combat, the beast acts during your turn.

Thus, we can see, that in the most recent iteration of game rules, the idea of two creatures acting simultaneously on a turn is endorsed, and possible in specific cases.

The following clause from the PHB Mounted Combat rules:
A controlled mount can move and act even on the turn that you mount it, seemingly, is a more fanciful, but less clear statement indicating the mount acts, simultaneously, on your turn.

As an aside, I enjoy "Puzzler" type questions. I agree that per the Mounted Combat rules a Controlled Mount is technically denied the Ready action.

Yet it is possible for a non-controlled horse, standing next it's rider, to have Readied an action to "Move up to the horse's speed, when it is mounted by the horse's rider".

So what happens to the Readied action when the horse 'transitions' from being an Independent Mount to a Controlled Mount?

The rules are a framework. The rules were created by non-omniscient and non-prescient humans..and are of course not 'Perfect'.

I think some posters are falling prey to the "Brainy Smurf Fallacy" of relying too much on : "Jeremy Crawford always says..." or "RAW is always consistent" types of argumentation. 🃏

Brainy Smurf was often wrong, and lead directly to the following UNICEF clip🃏https://www.youtube.com/watch?v=owo4f3QQBKc

[Tanarii]

A controlled mount "moves as you direct it" implies that the mount is sharing your turn (including substituting the use of its movement in place of the rider's), rather than acting independently on its own turn, as independent mounts explicitly act, separately on their own initiative.

It implies that it moves as you direct it. It doesn't imply timing.

The other two lines do have implications.

One reads most easily as it has its own initiative the same as yours, which has its own rules.

The other, without context, can easily be read to imply that it shares the turn with the rider. I'm not denying that taken alone, that implication makes sense. But taken together with the first, it doesn't.

[BigRedJedi]

It implies that it moves as you direct it. It doesn't imply timing.

The other two lines do have implications.

One reads most easily as it has its own initiative the same as yours, which has its own rules.

The other, without context, can easily be read to imply that it shares the turn with the rider. I'm not denying that taken alone, that implication makes sense. But taken together with the first, it doesn't.

Except that in multiple other situations (e.g. the Battle Smith's Steel Defender, etc.), the conditional for the pet/companion/animated porcelain goat to take their turn at the same initiative count, but subsequent to the player, is explicitly listed, but notably lacking in the discussion of mounts.

The last line of that paragraph, in particular, concerning moving and acting on the turn mounted, seems to indicate that the mount shifts its initiative and becomes more like a piece of equipment and less like an independent entity (especially with the further rule that intelligent mounts retain their own initiative), with the clarification about initiative matching the player's to prevent confusion or to make being on a mount less useful than being on foot.

[MaxWilson]

Except that in multiple other situations (e.g. the Battle Smith's Steel Defender, etc.), the conditional for the pet/companion/animated porcelain goat to take their turn at the same initiative count, but subsequent to the player, is explicitly listed, but notably lacking in the discussion of mounts.

The last line of that paragraph, in particular, concerning moving and acting on the turn mounted, seems to indicate that the mount shifts its initiative and becomes more like a piece of equipment and less like an independent entity (especially with the further rule that intelligent mounts retain their own initiative), with the clarification about initiative matching the player's to prevent confusion or to make being on a mount less useful than being on foot.

You make a good argument and I agree, but clearly we're not going to reach any kind of group consensus.

[BigRedJedi]

You make a good argument and I agree, but clearly we're not going to reach any kind of group consensus.

A toast! In honor of civil discussion, despite differing viewpoints, and for not devolving into the Dueling & Thrown Weapon thread!

Cheers!



P.S. A big thank you to everyone who is arguing in good faith, it's why I love this community.

[Sherlockpwns]

I'll weigh in as I've handled mounted combat extensively.

A quick breakdown of the text RAW as I have seen them.

First: The initiative of a controlled mount changes to match yours when you mount it.

This is here to ensure you DON'T end up with a mount at Initiative 16 and then you AND the mount at init 13 or something weird and even more disjointed (see intelligent mounts for that mess).

Second: It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge.

Pretty simple really, it can move and take one of those 3 actions (only). It can't take the attack action, for instance. Don't ask me about the whole dragon's breath thing, is it ONLY those 3 actions or is it those 3 actions by default? Who knows, it's never talked about.

Third: A controlled mount can move and act even on the turn that you mount it.

Still simple, you mount, it gets to move and act on the turn. I will explain this below, but what it's trying to get at is you both have the same initiative and the same turn, but do NOT act at the same time.

So why do I say this? RAW you are not going at the same time. There are very few instances where things move at the same time in ANY book, let alone just the PHB when 5e first came out (I don't think it existed at all in fact). It's always called out specifically too when you do see it. Since it is not called out directly what happens is you MOUNT and the init changes to yours. You give the horse (or fighter) a command and it does that action immediately following your turn as PART of your turn's initiative. The "can act on your turn" and the change to initiative are there to resolve the confusion of "what happens if my mount has already moved this round and I just reached him" - it gets to go again. Which is cool, but not really game breaking since it can only happen once (as dismounting does not reset its initiative, and after that it can't get two moves in a single turn).

So why even bother with mounts? Well, for one it's a huge boost to your mobility. You can charge in with a readied lance action, hit at 10ft, and away, no AOOs. Or you can wade in, stop, swing like a madman. Wait a turn, swing like a madman again, and disengage. No AOOs. You've gotten two rounds of attacks, the enemy only got one, and unless you're up against something REAL fast, they're probably not going to be able to reach you. You can just... shoot things and stay far away from them.

Is that more fun than going simultaneously? No, it really isn't. In my play experience being a roving terror on horseback is way more fun for the players. It's also a huge buff. Assuming you're really building for mounted combat, be that a steel defender, a ranger companion, a paladin mount, or a bard on a Pegasus, if you allow simultaneous movement it is a power spike that requires active DM intervention for most campaign settings. Like, a melee enemy at BEST can just ready an action to hit you when you approach and if they don't have ranged weapons, literally can not do anything. So the DM will spend all their time trying to tweak encounters to ensure more ranged enemies, more traps, more fliers, more small rooms - anything - to stop the player party from just kiting everything to death. I'm not saying this is BAD, I am just saying that if you open the pandora's box and end up with a party of 5PCs with mounts you'll spend a lot more time trying to come up with clever combat situations to challenge their mounted supremacy. Expect a lot of ladders and tight spaces and stuff, lol.

Not only that but RAW you can STILL kite-kill anything in the open that is slower than you and lacks the range you'll undoubtedly have, so letting the more fun rules play out just makes everything easier for the players and feels way less disjointed to play. RAW feels kinda... janky to play, quite frankly, but it's an ok-ish middle ground between giving PCs power and mobility and giving them super-powers.

Oh and I saw someone say that mounts die easily. They really don't. Again, I am not including buying a crappy warhorse without the mounted combatant feat, or even Find Steed Warhorses. I am talking about Find Greater Steed, A Steel Defender, a Ranger Companion, etc... between simply being able to redirect any attack on your mount to you AND they all have evasion, it takes some serious firepower to bring a mount down (usually in the form of making them save vs. restrained or similar AOE effects that are not dex saves, for any DMs dealing with mounts and struggling). All that attention on killing your mount is NOT attention on killing you... again meaning the DM usually has the raise the CR of the encounter just to account for one or two mounted players, let alone a whole group that wants to become the Mongol Horde.

Anyway, that's my 2 cents on mounted combat. RAW I think they move sequentially, at the PCs choice of which goes first. It's weird to play, but it makes sense for balance. Rules as FUN, do it the more logical "both at once" way and let the DM change the encounters to mess with you. I've played both ways and I much prefer the latter, but after many discussions with the groups doing mounted combat we all ended up agreeing that my interpretation of RAW is correct, albeit kind of ****. It's one of those "D&D" things where turn based system trumps any sense of reality. Like how moving diagonally is faster than moving in a straight line. It's just D&D logic :D (Yes I know the DMG has different rules than the PHB for this, though you have to wonder wth they were thinking by having two different rules for this at all!)

[greenstone]

Third: A controlled mount can move and act even on the turn that you mount it.

Still simple, you mount, it gets to move and act on the turn. I will explain this below, but what it's trying to get at is you both have the same initiative and the same turn, but do NOT act at the same time.

Is this allowed under your ruling?

Fighty McFightFace the Fighter, who has 30ft movement and Extra Attack (2 attacks), is standing 15ft from Horsey the Warhorse (60ft movement):

On Fighty's turn: Fighty attacks a target (Fighty takes the Attack Action), moves 15ft to Horsey, mounts, moves Horsey 120ft to another target (Horsey takes the Dash Action), then attacks a second target.

[Zhorn]

Third: A controlled mount can move and act even on the turn that you mount it.

Still simple, you mount, it gets to move and act on the turn. I will explain this below, but what it's trying to get at is you both have the same initiative and the same turn, but do NOT act at the same time.

Is this allowed under your ruling?

Fighty McFightFace the Fighter, who has 30ft movement and Extra Attack (2 attacks), is standing 15ft from Horsey the Warhorse (60ft movement):

On Fighty's turn: Fighty attacks a target (Fighty takes the Attack Action), moves 15ft to Horsey, mounts, moves Horsey 120ft to another target (Horsey takes the Dash Action), then attacks a second target.

Good scenario, highlights a section of the rules that you need to ignore to make the overlapping/simultaneous turns work.

Breaking Up Your Move
You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feet.

Moving between Attacks
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.

Again, if a DM wants to simplify the whole thing to being the rider uses the mounts speed on their turn, so be it. Just sounds like a ruling conflict to me.

You're allowed to break up you own movement on your turn before and after your action, and you can break up your attacks using your own movement... But the mount isn't you, and their movement speed isn't your movement speed.

The rules for breaking up movement on your turn explicitly states "your movement" and "your action". Even if inserting the mount's movement and action into your turn, it does seem like a conflict there to them break up your action with another creature's movement, or break up your movement with another creature's action.

To stay consistent here, the mount's and rider's movement and actions would be unable to overlap with the other's. You could try and mix it up with differing orders of:

movement(rider's) > movement(mount's) > action(mount's) > action(rider's)
action(mount's)> movement(mount's) > movement(rider's) > action(rider's)
action(rider's) > movement(mount's) > action(mount's) > movement(rider's)
(etc etc, I won't list all 24, but I hope the idea general idea is clear)

But it would bar out the Dash action from either rider/mount(A) if the other(B) was able to use an action to separate A's movement and action, since that would result if movement being broken up by another creature's action...

Filtering out such combinations to avoid such edge cases will leave you with A's movement and action and then B's movement and action (be that either rider or mount first or second), which boils down to simply having sequential turns rather than overlapping turns.

[Witty Username]

Yes, that's how I rule it too. The only exception would be Small Size Beastmasters riding their pet.

Why the exception for small size beastmasters?