Ankylosaurus

An ankylosaurus on the party's side is almost always a druid wild shape, a frontier lord's prize beast, or a contracted living siege weapon hauled overland for one specific job. The dinosaur is a PC if it is a wild shape. It is an NPC the party negotiates with through a handler if it is a trained beast.

Use the dinosaur for what it actually does, which is occupy ground. With Huge size, AC 15, and 68 HP, a parked ankylosaurus is a wall the enemy has to work around. Position it across the doorway, the bridge, or the chokepoint. Hostile creatures that try to push past have to either go around or shove a six-ton armored beast, which is not happening at most low-CR tables. The dinosaur buys the back-line casters two or three uninterrupted rounds, and that is why a party brings one to a fight.

The dinosaur cannot take orders past stay and go. Any complex behavior requires a Wisdom (Animal Handling) check from the handler. On a fail, the ankylosaurus does the dumbest thing in range, which usually means standing still and chewing. Do not punish the party for this, but do not let them treat the beast as a fifth PC either.

Make stabling and feeding a logistics problem the party has to solve in town. Forty pounds of fodder a day, a yard tall enough fences will not matter, and somebody to walk it. The ankylosaurus is the kind of asset that creates more story than it resolves.

An ankylosaurus is a Huge plant-eater that becomes terrifying when the party blunders between it and something it cares about. AC 15 and 68 HP give it enough durability to absorb hits without panicking, and it swings two Tail attacks each round at +6 to hit, dealing 9 damage per swing (1d10+4 bludgeoning) and knocking anything Huge or smaller Prone on hit.

The dinosaur's identity at the table is a wall of muscle that does not negotiate. Its 30 ft. walk speed is unremarkable, Passive Perception 11 is poor, and Int 2 means no flanking, no feinting, no clever switches. It picks one PC each round, the closest or the one who most recently hurt it, and commits to swinging until that target drops or moves away. The fight feels honest if the dinosaur is dumb and consistent rather than puppeted into smart targeting. A Prone target takes both Tail attacks at advantage on the next round, which is as close as beasts get to strategy.

Use terrain to make the encounter interesting. The slow 30 ft. walk speed turns a flat plain into a war of attrition the party wins in five or six rounds. Put the ankylosaurus in a narrow canyon, a riverbank, a corral the party blundered into, or guarding a nest. Now the slow speed is a feature, because the dinosaur sits between the party and where they need to go. The squishy members have to cross the kill zone or talk the rest of the group into a careful flank.

Retreat happens when the dinosaur is below half HP and food is on the ground. It does not pursue past the immediate area of its nest or grazing ground. Drop a horse or an unconscious henchman, back off, and the encounter ends with the party shaken but alive. Describe the tail before the swing. A heavy thump as the club drags across stone, then the swing, then the save.