Bulette

A bulette on the party's side is a stretch but not a forced one. Frame it as a partial bond: a ranger with the right downtime spent acclimating it, a druid who shared a kill with it, a dwarven mining outfit that feeds one to keep it patrolling their tunnels. The bulette is Int 2 and Unaligned, so this is not friendship, it is a feeding arrangement. The moment the party stops being more useful than a deer, the deal is off.

In a fight, the bulette is a missile the party aims and then gets out of the way of. Have the handler spend an action to point at a target, and on its turn the bulette burrows to within 15 feet and uses Deadly Leap on whoever is standing where you pointed. DC 15 Dex save, 19 bludgeoning, prone on a fail. Follow up with two Bite attacks at +7 to hit, 17 piercing each. The bulette will not distinguish between the goblin chieftain and the goblin chieftain's hostage, so the handler had better point carefully.

The exit is the meal. Once an enemy goes down and is bleeding, the bulette starts eating, and combat is over for it whether or not other enemies are still standing. Tremorsense out to 120 ft. makes it a perfect scout in the half-hour before the party rests. It will warn on anything walking or digging within range, usually by going stiff and growling at a wall. After the encounter, it wants a horse-sized portion of meat, and if the party didn't bring one, it goes hunting on its own.

Give the bulette a name only the handler uses. The rest of the party should be a little afraid of it the entire time.

A bulette is an ambush from below. The encounter doesn't start when the party sees it, it starts when the party fails to see it. With 40 ft. of burrow speed and Tremorsense out to 120 ft., the bulette has been tracking the party's footfalls for half a mile before it surfaces. Run the approach as a sound design problem: pebbles vibrating, a horse going skittish, then the ground under the wagon detonates.

Open with Deadly Leap as the surface action. Spend 5 feet of movement to jump up to 15 feet into a space holding one or more Large or smaller creatures, DC 15 Dex save, 19 bludgeoning on a fail and the target goes Prone. That's a clean opener that knocks two PCs flat and puts the bulette in the middle of the formation. From there, two Bite attacks at +7 to hit for 17 piercing each is reliable damage on a now-prone target. The bulette is dumb (Int 2) but it understands "things on the ground are easier to bite", so once it has a prone target it stays on that target until the next round.

Mobility is the trick that keeps the fight from devolving. The bonus action Leap jumps 30 feet for 10 feet of movement, which lets the bulette disengage out of melee, reposition over a fallen caster, and then bite again next round. Use it when the fighter and the paladin both have it surrounded. If the fight goes badly, burrow straight down. The party cannot follow at 40 ft. of burrow speed, and the bulette will be back in twenty minutes hungrier and angrier.

Run it like a shark, not a soldier. It is hunting, not winning. If the party drops a horse or a pack mule, the bulette will absolutely take the easier meal and leave, and the players will remember that for the rest of the campaign. Lean on the Tremorsense range when the party tries to stand still and hide. Standing still on dirt does not work.

Telegraph the approach before the surface attack. A line of dust kicking up across the road, a horse refusing to move, one round of "you feel the ground hum." The fight starts in the round of warning, not in the round of damage.