A large ape-shaped subterranean desert-dwelling beast that excavates tunnels with iron-hard claws and devours other cave-dwelling creatures with its expandable jaws. The Ambull is originally native to the death world of Luther McIntyre, but nowadays can be found across the Imperium thanks to well-meaning Magos Biologis of the Adeptus Mechanicus spreading the creatures around in attempts at domestication (apparently they taste delicious when properly cooked). Unfortunately, though not unsurprisingly, a dead'ard predator whose main shtick is tunneling proved almost impossible to contain or control, with the Ambulls almost always managing to escape into the wild and become an exceedingly dangerous pest on any world they end up infesting. Why nobody thought to simply lobotomize them like Grox is anyone's guess.
Exactly how tough Ambulls are has varied a bit over the course of their appearance history, however. Back when Ambulls were just a bit of old background fluff, Ciaphas Cain and a squad of Valhallans were able to wipe out almost an entire nest full of them with minimal casualties, and the one guardsmen who was injured by the Ambulls survived. The recent reintroduction of Ambulls to the setting has significantly beefed them up, with a single Ambull being the focus of a whole expansion to Blackstone Fortress, making it out to be a monster that would take a whole team to bring down. Its larvae, the Borewyrms, aren't quite as lethal on their own but can be just as dangerous in large numbers.
On Necromunda, they apparently like to make robotic versions of the Ambull called Ambots (or if you want to be technical, "Luther Pattern Excavation Automata"). The Ambull's brain is placed inside a mechanical body intended for mining purposes and then fitted with restraining mechanisms to keep its violent temper in check. Of course, the more creative thinking Hive Gangers found a new use for the hulking Ambot after figuring out how to remove its restraints; namely as a nigh unstoppable killing machine. Given its powerful tunnelling claws with built-in meltaguns, it's as much of a force to be reckoned with as its fleshy counterpart. For God’s sake AdMech! If normal humans can legally mass produce shit like this, where the fuck are our endless armies of killing machine-cyborg-gorilla-bug monsters for the Imperial Guard? Imagine a hive dedicated to producing weaponized ambots...now put those bots against Tyranids and Orks and Chaos.
From the description of the Ambull's physical appearance, and their knack for tunneling, there's a certain resemblance to the Umber Hulk, a Dungeons & Dragons monster that predates the creation of 40k.
Ambulls first appeared on the table top during the Rogue Trader days of yore. At the time they were intended to be played on the tabletop by a neutral third player, or weaponised by taking them as part of your army at which point they moved randomly and charged everything within range. There was the rather amazing upgrade for them whereby you could Deep Strike them onto the battlefield via teleport, essentially inflicting them on the enemy rather than deploying them with your own troops; which would be stupid.
In 2019 the Ambull finally returned to the tabletop, in both regular and robotic varieties, via expansions to Blackstone Fortress and Necromunda respectively. And, just like Blackstone Fortress, there are 40k rules for the Ambull.
His statline's pretty monstrous with S/T6 and 7 wounds that regenerate each round with a 3+ save, while his claws are AP-3 Dd3, meaning this fucker can rip through marines like paper. While he has a rather decent M6, he can dig underground for a turn and then reappear wherever he wants the next movement phase. This isn't quite as scary as Genestealer Ambush, but there's nobody that'd want a giant killer bug near them. It also means that if it's in over its head it'll always have an escape plan.
Like Spindle Drones, Ambulls have the keyword Unaligned, so they can be used in any army, but only for matched or narrative play. They would serve well as a thematic hazard or an objective to kill. Maybe give it to a third player to control.
An Ambot of House Escher, referred to as "Mr. Chompers"