Mule

A mule is the party's first ally and the one they will mourn the longest. CR 1/8, 11 HP, AC 10, walking speed 40, no listed actions. Mechanically it is a beast of burden with Strength 14 (Str save +4, presumably reflecting its build), and that's the entire stat block. Run it as the wagon, the kit, the supply line, and occasionally as a piece of soft cover the rogue ducks behind during the bandit fight.

The mule's job is logistics. Standard 5e carrying capacity puts a mule at roughly 420 lb. of hauled gear without strain, double that as a draft animal. That covers the wizard's library, the cleric's tent, the fighter's spare plate, and the rogue's collection of chalk, lockpicks, and unfortunate trophies. When the party makes a roll to do something physical with the mule (haul a log, pull a friend out of a pit, ford a river with the wagon), the +4 Str save is what you call for. Outside of combat, the mule is a permission slip: the party can carry the gear they actually want to carry without the petty bookkeeping of encumbrance.

In combat, the mule does not fight unless cornered. It panics. Use a Wisdom check from whoever holds the lead rope, DC scaling with the threat (DC 8 for a wolf at distance, DC 14 for a dragon overhead). On a failure, the mule bolts at 40 ft. in a straight line and the party loses the wagon for a scene. On a success, it stands and the party can plan around it. If something does close to melee, narrate a kick at +4 for 1d4+2 bludgeoning as an Improvised Attack and move on. The mule is not a fight. It is the reason the fight has stakes.

Name the mule. Players who would not blink at a dead bandit will negotiate with a kobold for a dying mule. The mule is the cheapest emotional investment a campaign offers.

A mule as adversary is a stretch the GM has to commit to with a straight face. The most plausible setup is a haunted, possessed, or polymorphed animal: a hag has cursed the village's pack mule, a necromancer's failed experiment is shambling toward town with a saddle still on, or a transmuted PC enemy is hiding in mule shape and waiting for an opening. The encounter is comedy that turns serious in the second round.

Run the fight as the mule's panicked aggression rather than as tactical play. Speed 40 lets it kick (Improvised Attack at +4 for 1d4+2 bludgeoning), trample anything Tiny or Small that gets behind it, and bolt away from the rogue who is trying to grapple it. If the mule is possessed, give it Darkvision and one bonus attack, and have whoever's controlling it speak through it in a voice the party recognizes from a previous session. The horror beat is the mule using a dead NPC's words.

The fight ends quickly because 11 HP doesn't last. The aftermath is the scene. Was the mule the friendly one from the inn? Did the curse pass to whoever lands the killing blow? Is there a second mule in the same caravan still waiting? Don't roll for any of this on the table. Just narrate one ominous thing and move on. The image of a dead mule with a familiar saddle is enough.

End the scene with the mule looking up at a player and naming them by the wrong name. Use the name of a dead NPC the party knew. The mule does not need to do anything else.