Ankheg

A tame ankheg is a stretch, but stretches are the fun ones. The frame: a frontier farmer has bonded with one as a tunneling beast for irrigation work, or a dwarven mining outfit raises ankheg broods to chew exploratory shafts through bedrock. The Tunneler trait is the entire reason the relationship exists, and the party gets access to it because the owner owes them a favor or hired them to escort the brood across hostile country.

Don't try to make the ankheg a combat partner. With Intelligence 1 it follows simple commands at best (down, up, here, away), and the Bite is friendly fire waiting to happen because of the Grapple and Acid rider. Use it for one specific problem: digging through a wall, breaking into a barrow, getting the party under a city gate without being seen. The 10-foot-diameter tunnel it leaves is wide enough for a horse, which is the gameplay hook. Acid Spray is also off the table for tactical use unless the party is willing to accept that anyone in a 30-foot line eats 4d6 acid.

When the fight does start, position the ankheg at the back of the party and let it Burst-and-Bite a single big target the players have flagged. Once it's fed, it loses interest and goes back to digging. Run it like a working animal that happens to weigh as much as a draft horse and bleeds acid.

Give the ankheg a name and a handler. The handler does the talking; the ankheg makes the table feel uneasy.

An ankheg is an ambush from below. The whole encounter design is geared around the Tremorsense 60 ft. and the burrow speed: the party is walking through a field, then a horse is gone, then the wagon is missing a wheel. Set the scene as a normal day of travel for at least one paragraph before you call for initiative.

Open the fight with the ankheg already underground and adjacent to whoever is heaviest, because the Bite uses Strength against AC and the grapple matters more than the damage. On the surprise round, burst up and bite: +5 (with Advantage on the surprise), 10 slashing plus 3 acid, and a Large or smaller target is Grappled (escape DC 13). Now the ankheg burrows back down with the grappled PC dragged along, because the Tunneler trait says it can chew through solid rock at half speed. That's the nightmare beat. The party's friend is being eaten underground and they have ten feet of fresh tunnel to follow.

If Acid Spray is up, save it for when at least three party members are clustered. The 30-foot line at DC 12 deals 14 acid on a fail, which is meaningful at low levels but becomes a single-round commitment of the action. Don't burn it on one target. The bite-grapple loop is the engine; Acid Spray is the panic button. With Intelligence 1, the ankheg has no real morale check, but a swarm response from the village (torches, pitchforks, a cleric with Create Food and Water) should drive it off if the party is losing badly.

The horror lever here is sound. Describe the muffled thump of chitin moving under the floor of the inn before anyone sees it. By the time someone says "what was that," the floor opens.