Triceratops

A triceratops on the party's side is a druid's wild shape, a beast of burden hauled out of a lost-world adventure, or a lizardfolk gift to an ally tribe. The framing matters because the triceratops cannot be ridden into most settled places (Huge, gates do not fit, peasants flee). The ally version works best in jungle, savanna, or wasteland campaigns where the party is far from civilization.

In combat run the triceratops as heavy cavalry. The opener is the charge at 50 ft. Position it 30 feet behind the front line and let it run through the enemy formation on round one, knocking two or three prone, then have the rogue and paladin clean up while it wheels for a second run. Multiattack gives two Gore actions per turn for sustained pressure.

Out of combat the triceratops is a wagon with a frill. It hauls gear no horse could lift, fords rivers without panic, and intimidates predators away from camp. Riding requires a saddle the party probably had to build and an Animal Handling check at DC 13 to keep it pointed. A druid using wild shape gets the same body without the saddle problem but loses spellcasting while in form.

The triceratops eats constantly and produces an amount of waste that affects camp choice. Make the pet logistics part of the bit. Players who own a Huge dinosaur should feel the cost of owning one. The moment they have to leave it behind to enter a city should land like leaving a friend at the gate.

A triceratops is a CR 5 freight train. Huge, 114 HP, AC 14, Strength 22, walk 50 ft. The Gore action gives +9 to hit for 2d12+6 piercing damage. If the triceratops moved 20 feet or more straight at the target before the hit, the target takes an extra 2d8 piercing damage and is knocked prone. Multiattack gives two Gore attacks per turn. The creature is a mood with horns.

The opener is the charge. A 50 ft. walk and a 20 ft. charge requirement means the triceratops covers a lot of ground in one round. The party that sees it coming has time for one ranged attack each before they scatter. The prone knockdown from a charge is the lever. A downed PC eats a second Gore attack on the next turn, which is real damage even at tier 2. The triceratops charges once, wheels (a full action to turn that bulk), and either charges again or gores whoever is at its feet.

Run multiple triceratops if the encounter is meant to be the climax. A herd of three at CR 5 each is a terrain-changing fight. The party cannot stand in one place because the ground is moving. Pick targets by who is loudest and closest. Triceratops do not have favorites; they have whatever is in front of them.

Once bloodied at 57 HP, a lone triceratops disengages back to its herd or its young. A defending bull dies on the field. Decide which it is before the scene starts. Have the ranger roll Nature DC 13 to spot a calf in the brush before the fight begins. The party that sees it can avoid the encounter. The party that does not has just learned why the bull was so angry.