Rhinoceros

A rhinoceros as a willing ally is unusual but not absurd. A frontier rancher's prize bull that pulls a wagon alongside a familiar handler. A savannah tribe whose elder rides a war-rhino into ceremony. An old druid who has bonded a single rhino across decades.

Use the rhino as a battering ram and cargo capacity, not as a combatant in nuanced fights. It pulls a heavy wagon faster than most horse teams, carries two riders at uncomfortable speed, and charges a portcullis, barricade, or hostile formation with a handler coaxing it. The 40-foot walk and Strength 21 do most of the work. Treat the Gore as a contested Strength shove that knocks Medium creatures Prone.

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

Have the handler whisper to the rhino once before any charge, in a language the players do not understand.

A rhinoceros is a charge in a straight line and very little else. CR 2, 45 HP, AC 13, Strength 21, walk 40 feet. It has a single Gore attack at +7 doing 2d8+5 piercing. If the rhino moved 20 feet straight toward the target before the hit, the target also takes an extra 2d8 piercing and is knocked Prone. The encounter is about momentum rather than damage rolls.

Open at distance with line of sight. Passive Perception 11 means the rhino notices a moving party at about 60 feet in clear terrain, and once it notices, it commits. The classic opening is a savanna crossing: the rhino lifts its head from a wallow and starts to run. Give the players one round of warning. Anyone who Dashes makes 60 feet and is probably out of the cone. Anyone who readies a brace action can meet the first charge. The party that stands and discusses eats it.

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

Retreat happens when the rhino has driven the threat from its territory. Once the party is 100 feet away or behind cover, it huffs and turns away. Beasts don't chase past the boundary of what they defend.

Have the rhino break a sapling in half during the warning round.