Giant Elk

A giant elk is a numinous escort, not a combat companion. It's a Huge Celestial with Neutral Good alignment, 60 ft. of walking speed, and the bearing of a forest god that condescended to walk beside the party for an afternoon. Frame the meeting as a gift: a druid's prayer answered, a sacred spring crossed at the right moment, a dying king's last hunt forgiven. The elk arrives, bows its rack of antlers once, and walks beside the party for the duration of one specific journey.

It speaks Celestial and understands Common, Elvish, and Sylvan, but it cannot reply in those languages. Lean on that. Questions get answered with movement. Stop, turn, look at a particular tree, paw the ground twice. A ranger or druid in the party can interpret, and that interpretation is part of the scene's pleasure. Resistance to Necrotic and Radiant tells you what kind of country it has been walking through, so let it react to undead the way a horse reacts to a snake.

In a fight where the party rides it or follows it into a charge, Ram is plus 6 for 2d6+4 bludgeoning plus 2d4 radiant. If the elk moved at least 20 ft. straight toward the target, add another 2d4 bludgeoning and knock it Prone. The elk is a 42-HP mover at AC 14. Use the speed. Charge through a line of cultists to break them up, then carry a wounded PC out of the kill zone at 60 ft. per round. Darkvision 90 ft. lets it lead the party through a black wood without torches. The elk will defend itself by ramming, but if the situation turns into a stand-up fight, the right call is for the elk to bear the wounded clear and return to whatever realm sent it.

Have the elk leave by simply walking into a stand of birches and never coming out the other side. Don't narrate a flash, don't narrate hooves on cloud. The party should look up and notice it is gone.

A hostile giant elk is a tragedy with antlers. The framing has to do most of the work. Try one of these: the elk has been driven mad by a nearby blight, a hag has bound a fey collar to its throat, the party trespassed on a sacred glade and the Celestial guardian has decided they are the rot to be cleared. In every case, the elk is not evil. It is wrong, and the party will realize it by the second round.

Ram is plus 6 for 2d6+4 bludgeoning plus 2d4 radiant, with an extra 2d4 bludgeoning and Prone on a charge from 20 ft. Open with the charge from across the clearing, blow through the front line, and circle for another pass. The elk will not stand still in melee. It uses its 60 ft. speed to keep the party reacting, lowers its head at full sprint, and breaks contact the moment the wizard starts a long-cast spell. Darkvision 90 ft. and 14 passive Perception make it hard to ambush in the dark.

The right ending is not a kill. Let the party notice the collar, the brand, the foam at the mouth that shouldn't be there. A successful Animal Handling or a Calm Emotions ends the encounter. Killing the elk should feel like a failure even if it was the only option.