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. With 50 ft. of walk and Huge size, it covers ground faster than any human-shaped enemy can reposition, and 136 HP at AC 13 means it can soak a round of focused fire that would drop the party fighter twice over. The lack of ranged or rider-friendly options means 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. It is Huge, 50 ft. of walk speed, 136 HP at AC 13, and absolutely no plan beyond eating whatever it locked onto first. Run it as a stat block whose 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 Mage Hand at it 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 the terrain takes ranged advantage off the table and forces somebody into reach. With Perception +4 and Passive 14, 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 stat block in this entry has no listed actions, so adjudicate the bite the way the SRD T. rex traditionally bites: a single massive attack against the chosen target each round. Twelve points of difference between Str 25 and the average PC's Athletics means a successful grab is a held PC and a 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. Make the horse the one nobody named.