Ambull
The ambull were typically known as large ape-shaped beings with a scaly sort of skin, large mandibles and seemed to appear out of nowhere(Creed), furthermore these pincery fuckers are originally thought to be a warp creature, not a demon as such but more of a... thing, anyhow the imperial guard went around trying to domesticate these things, as one would expect a 6-7ft man-crushing monster would make a particularly good guard dog, however these guys were having absolutely none of it and just tunneled the fuck around, mincing anything that they disliked, so the Imperial guard decided to leave them alone.
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.