Deer

A deer is not a combatant, a mount, or a quest-giver. It is a piece of the world the party has decided to care about, and the entire trick of running one well is making the players feel that care without asking them to roll for it. The stat block is 4 HP, AC 13, walk 50 ft., Passive Perception 14, with a Ram action at +2 for 2 damage (1d4) and Agile trait that prevents opportunity attacks. There is almost nothing here mechanically. Treat the deer as a noun, not a verb.

The most common ally framing is the druid's animal companion, a Find Familiar reflavor that the DM has agreed to honor as a deer rather than the standard list, or a wild creature that the party has fed once and now follows them at a polite distance. Another is the courier role: a noble's pet hart that the party is escorting back to the estate, or a sacred deer the temple has asked them to walk to a shrine without harm. Whatever the frame, the deer cannot defend itself, so the players are responsible for it. That responsibility is the whole gameplay loop.

In a scene, the deer's value is the 50 ft. walk speed and the Perception. Send it ahead on a forest trail and use its body language to telegraph what is around the next bend. Ears up means a sound the party has not heard yet. Tail flicking means it is nervous. A bolt straight back the way you came means a predator is close and the party has about ten seconds to get into cover. None of this requires a roll. The DM narrates and the players react.

Have the deer get hurt once. A snare, a lucky arrow from a bandit, a wolf that the party did not see in time. Let them choose whether to spend a healing spell on a beast that will not ever thank them. The campaign that gets a yes there is a different campaign than the one that gets a no.

A deer as adversary is a stretch, and you should commit to the stretch openly rather than fudge it. The framings that work: a stag possessed by a fey trickster who finds the joke of charging a paladin worth the risk, a herd-leader rutting in autumn whose territory the party has blundered into, or a sacred deer the party has been told under no circumstances to harm, now actively in their way at a chokepoint. The first two are real combat. The third is a moral encounter the players have to solve without violence.

Mechanically there is almost nothing here. AC 13, 4 HP, Ram at +2 for 2 damage. The 50 ft. walk speed and Agile trait mean the deer can disengage at will from anyone without a horse or a Dash. Use it. The deer should never be cornered into a swing because the moment it takes any damage at all, it dies, and a deer dying in one hit is not the encounter you are trying to run.

The right shape for this scene is a chase or a standoff, not a brawl. The deer charges, the party scatters or holds ground, the deer breaks off, the deer comes back. If the party kills it in one hit, sit with the table for a beat before describing the body. Sometimes that beat is the encounter.