Rules for running on giant monsters?

[Frostthehero]

The thread is pretty self explanatory, it's just that I have no idea about the rules. The closest thing that I can find is a grapple check, but that seems stupid to me.

Is there a RAW entry on this, or do we have to make stuff up? There are so many rules that have to be considered. The most obvious is how do we initiate this? Secondly, what are the penalties for losing? Third, what happens if the monster someone is running on begins to shake, or makes an attack? What are the rules for getting a runner off of you?

The point is, I need these questions answered. My players are begging me for rules on this.

[Twurps]

Use a grapple check to run? that sounds counter-intuitive to me, as the point of grappling is to stay put.
I would use the 'difficult terrain' rules on the monster, or whatever house-rules you normally use for running through stuff like long grass, or uphill. (The movement rules are rather digital, meaning you can either run, or you can't, our group usually uses some other degrees of movement in between)

If the monster starts to move, anyone on it needs to make a balance check to stay upright, with the DC modified to the type of move they're taking. (Running at a moderate speed might actually make this easier then when you're sitting on it for example, as it limits the time you're in contact with the monster)

[Curmudgeon]

The basic rule is: you can't do it.
Ending Your Movement

You can’t end your movement in the same square as another creature unless it is helpless.

[Maglubiyet]

What type of creature? What are the relative sizes of the party and the monster?

Are we talking about a human jumping on an ogre's back or pixies running laps on an ancient red dragon?

[SiuiS]

RAW only applies where there isn't an obvious solution for it.

If a monster is big enough to be a surface someone can stand on, you stand on them. The end. If they're moving, you roll balance. If they are tall, you roll climb.

Grapple is a combat action. Grapple is not grabbing; it is holding the other still by engaging them physically. You don't grapple the floor as you walk. You don't grapple the wall as you climb.

[IZ42]

My RL GM actually had this problem when I jumped onto the back of a (Young?) Adult Black Dragon to deal with a Hobgoblin. He ruled it as Difficult Terrain, but didn't have me do anything special, at least until the dragon barrel rolled, where I had to make a Grapple Check to hang on(which I of course, failed)