Rhinoceros

A rhinoceros as a willing ally is unusual but not absurd. The natural framings are a frontier rancher whose prize bull will pull a wagon if a familiar handler walks alongside, a savannah tribe whose elder rides a war-rhino into ceremony and battle, and an old druid who has bonded a single rhinoceros across decades and treats it as a stubborn dog. The party borrows the animal for a specific scene where the answer to the problem is forty feet of moving stone.

Use the rhino as a battering ram and as cargo capacity, not as a combatant in nuanced fights. It can pull a heavy wagon faster than most teams of horses, carry two riders at uncomfortable speed, and (with a handler nearby coaxing it) charge a portcullis, a barricade, or the front rank of a hostile formation. The 40 ft. walk speed and the Strength 21 do most of the work; treat the gore as a contested Strength shove that knocks medium creatures prone and sends them ten feet sideways.

The rhino does not understand subtlety, friend-or-foe in melee, or the concept of stopping mid-charge. Once it is pointed and slapped on the rump, it goes. If the party's plan requires the rhino to charge into a crowd that includes their own paladin, the paladin is taking the hit too. Plan accordingly.

Have the handler whisper to the rhino once before any charge, in a language the players do not understand. The relationship is older than the party.

A rhinoceros is a charge in a straight line and very little else. Strength 21, walk speed 40 ft., 45 HP, AC 13, and a stat block with no listed actions means the encounter is about momentum rather than damage rolls. Run it as a freight engine: pick a target, point the head, and go. Use the basic rules' shove or grapple for the gore-and-trample, or rule a 2d6+5 bludgeoning hit on a contested Strength save, and tell the table that is your call.

Open at distance with line of sight. Passive Perception 11 means the rhino notices a moving party at maybe sixty feet in clear terrain, and once it notices, it commits. The classic opening is a savanna crossing, the rhino lifting its head from a wallow, the GM saying "it starts to run". Give the players one round of warning. Anyone who Dashes makes 60 ft. and is probably out of the cone; anyone who readies a brace can drop the rhino's first charge into a held action. The party that stands and discusses eats the charge.

The rhino's weakness is its turn radius. If the party splits sideways, the rhino runs past, takes 80 ft. to slow and turn, and has to pick a new target. A clever rogue can keep the encounter at one charge per round indefinitely with proper spacing. Trees, boulders, and ditches all break the line of charge, and a rhino that loses momentum is just a Large grumpy beast at AC 13 that eventually goes down.

Retreat happens when the rhino has driven the threat off its territory. Once the party is a hundred feet downwind or behind cover, it huffs, paws the ground, and turns away. Beasts do not chase past the boundary of what they were defending.

Have the rhino break a sapling in half during the warning round. Players who watch a tree snap tend to take a charge seriously.