Goat

A goat on the party's side is a pack animal, a druid's wild shape, or the cleric's holy companion in a pastoral cult. The most common framing is mundane: somebody bought a goat in the last village because it carries gear, gives milk, and eats anything. The party named it after one session and now they can't sell it.

In combat, the goat is not a combatant. AC 10 and 4 HP means a single arrow drops it, and there's no actions block to give it teeth. Keep it tied up at the camp, or at the back of the marching order behind a real fighter. If the players insist on pointing it at an enemy, the goat charges with its 40 ft. walk speed, headbutts for whatever the GM rules an improvised attack does, and then dies to the response. Do not let this happen by accident.

The value of the goat is out of combat. The 30 ft. climb speed means it can reach a ledge the rogue can't, and a rope knotted around its body solves several traversal problems. Darkvision 60 ft. and Passive Perception 12 means it notices the wolf at the treeline before the watch does, and it bleats. A bleating goat at 3 a.m. saves more parties than initiative does.

Have the goat eat a plot-relevant note in the second session it's around. The players will laugh, and then they'll spend an hour digging through the dung to recover what was on it.

A goat is not a fight, it's a complication. CR 0, 4 HP, AC 10, no actions block at all in the SRD. If the players actually swing on a goat, it dies to the first hit and nobody feels good about it. The role of a hostile goat at the table is to make the situation worse: it's blocking the path, it's eating the spell scroll, it's headbutting the wizard down a staircase that the wizard had no business being on in the first place.

Run the threat as terrain. Walk speed 40 ft. plus a 30 ft. climb means a goat is faster than a PC in armor and can be on a ledge the party can't reach. Passive Perception 12 and Darkvision 60 ft. mean it sees the party coming in any light. The mountain herd that lives above the road is not aggressive on purpose, but a goat dislodging a single rock down a scree slope onto the party is a Dexterity save, not an attack roll. Use the goat to set up the hazard, then let the hazard do the work.

The other angle is the herd. One goat is a punchline. Twenty goats stampeding through a market while the party tries to catch a thief is a chase scene. Run that as a skill challenge, not as initiative. Athletics to vault a stall, Animal Handling to part the herd, Acrobatics to keep your feet when a billy plants itself in your shins. Nobody rolls to-hit on a goat in this scene and the table should not want to.

If a player insists on attacking the goat for its hide, let them. Then have the herder show up with a tally and an opinion. The bill is more interesting than the combat round.