Leveling of normal creatures

[Crimsonguard]

As we all know normal creatures can indeed level up just as any player character.

As of today I will (probably) get a warhorse and I plan on investing some resources in this , such as getting it armour and taking the mounted combatant feet. I asked my DM whether it would be possible for it to level up and naturally he agreed. Problem is , there is no XP table for warhorses. Can someone offer some assisstance?

[calebrus]

Advancing an animal seems to be the sole province of the Beast Master in 5e.
If you guys do decide to do something, you have to make certain that whatever you choose is strictly inferior to what the BM gets for his companion, which is: its own HD or 4*level (whichever is greater) and + (your) proficiency bonus to attack, damage, trained saves.

And that's basically bottom of the barrel as far as combat ability goes, but anything more and it's basically adding another PC to the party.
The Mounted Combat feat allowing you to force the attack on you instead of your mount (and giving it improved evasion) basically makes leveling a mount redundant anyway. Your lowly horse will survive just fine if you have that feat.
I think that's why there are no rules for it, because it throws the Party/XP balance calculations out the window to allow it, and it's kind of redundant anyway.