Tyrannosaurus Rex

A T. rex on the party's side is a stretch with one workable framing: a druid PC has Conjure Animals, Polymorph, or a long-running Awaken plot, and now the party has a Huge dinosaur escorting them through somewhere it does not belong. The dinosaur is unaligned. It is not loyal. It tolerates the druid because of the magic and the food, and the second the magic ends or the food runs out, it leaves or eats whoever is closest.

Use it as a wrecking ball, not a tactician. Send the T. rex first into a fortified gate, a line of orcs, a cluster of wagons. Walk 50 ft. and Huge size means it covers ground faster than any human-shaped enemy can reposition. 136 HP at AC 13 means it soaks a round of focused fire that would drop the party fighter twice over. The party stays out of its melee bubble, gives it a target, and lets it work.

Out of combat the dinosaur is a logistics nightmare. Towns will not open the gates. Bridges will not bear the weight. The party cannot stable it. Trees will be eaten. Plan one scene per session that exists purely so the players have to solve the problem of where to park their dinosaur for the night.

Set the duration of the spell or pact before the session and do not extend it. When the magic ends, the T. rex blinks, decides the party is no longer interesting, and walks off. Do not negotiate the goodbye.

A tyrannosaurus rex is a single-target apex predator with the silhouette to match. Huge, 136 HP, AC 13, walk 50 ft., passive Perception 14. Multiattack gives one Bite and one Tail per turn. Bite deals 4d12+7 piercing and grapples Large or smaller creatures (escape DC 17). Tail deals 4d8+7 bludgeoning and knocks Huge or smaller creatures prone. The entire personality is forward momentum. It does not maneuver, it does not retreat, and it does not stop chewing the one PC it decided was lunch.

Set the geography first. A T. rex in an open field is a chase scene, not a fight. The party will attack from 120 feet and pepper it with arrows for an hour. A T. rex bursting through trees, into a canyon mouth, or up out of a riverbed is a real encounter, because terrain takes ranged advantage off the table and forces somebody into reach. Passive Perception 14 means the dinosaur tracks by sound and movement, so a party that holds still has a window to reposition.

Pick a target before initiative and commit. The T. rex closes 50 feet on round one toward whoever made the most noise (the cleric calling out warnings, the bard playing a horn, the fighter shouting orders). Once adjacent, it does not move off. The Bite has reach 10 ft. and grapples on hit, locking the target into sustained damage from both Bite and Tail next turn. A successful grab is a held PC and slow death by chewing.

A T. rex does not flee. It is too stupid to run from a wound. Fight it to zero or scare it off with a clearly larger creature in line of sight. If the party wants to drive it away, they have to make the noise come from somewhere else: a second herd, a falling tree, an illusion of a larger predator. Otherwise it eats until killed.

Have a horse die first. Players hate it, take the dinosaur seriously after, and the round of chewing buys the rest of the party a turn to position.