Chimera

A chimera as ally is a stretch, but it works as a half-tame mount belonging to a hill giant chieftain, a cult leader who fancies herself a beast-rider, or a wizard who botched a polymorph and now keeps the result on a chain. The party has either inherited it through plot, won it by killing the previous handler, or borrowed it for a single flight. It is not a friend. It is a hungry vehicle.

In combat on the party's side, the chimera will eat the wrong target if you let it. Have whoever is handling it make a Wisdom check or an Animal Handling roll each round to direct the heads. On a fail the chimera picks the freshest blood it can see, which is often a wounded player, and goes for them with its bite. On a success the chimera flies where directed and uses Fire Breath on the marked enemy, then closes for Multiattack. Ram-then-Bite is the combo to call out, since prone targets give the bite advantage and bump damage from 11 to 18.

Outside combat the chimera is a logistics nightmare. It eats horses, will not tolerate the party's familiar, and the goat head bleats at all hours. Every long rest with the chimera nearby costs a ration roll or some other complication.

When the party finally needs to be rid of it, let it go on its own terms. The chimera flies off mid-fight to chase a flock of goats and never comes back. That ending is funnier than killing it.

A chimera is an aerial ambush predator with three heads that all want to bite the same person. Run it that way. The 60 ft. fly speed and Perception +8 (passive 18) means the chimera spots the party first, picks the squishiest member it can see, and dives. Open the encounter in the air with Fire Breath if it's available: 15-foot cone, DC 15 Dex save, 31 fire on a fail, and the recharge means you can get it back inside three rounds. Center the cone on the cleric and watch the party's plan reshape itself.

After the breath, switch to Multiattack. One Ram, one Bite, one Claw, with the option to swap the Claw for a recharged Fire Breath. The Bite has a hidden teeth: 11 piercing on a normal hit, but 18 piercing if the chimera had advantage on the roll. So the priority is to set up advantage. Diving from 30 ft. up, attacking a prone target, attacking a target the players have flanked themselves into, anything that triggers advantage doubles the bite's damage dice. Ram is the setup tool: 10 bludgeoning and a Medium-or-smaller target is knocked prone, which sets up advantage on the bite next turn from a melee attacker.

The chimera has Int 3, so it does not coordinate with allies, count rounds, or recognize tactical surrender. It does recognize pain. Once Bloodied, it stops engaging the front line and flies straight up out of melee reach, breathing fire on whoever is left if Recharge cooperates. Reaching the chimera then requires the party to climb, fly, or shoot, and the chimera regains range every turn at 60 ft. fly speed.

If the party kills the chimera and stops there, they are missing the point. Three heads means three ways the chimera was hunting. Find the lair on the cliff face and put a clutch of chimera young inside, plus the bones of the last group who tried.