Encountered them in a depth 5 Pthumerian chalice dungeon. Not too threatening. Can only attack forwards and are shit at turning once you run past them. First I thought they are the swamp things from Nightmare frontiers, but these look slightly different.
For anyone wondering who doesn’t want to go to the reddit source:
Labyrinth Moles.
They are reportedly the rarest mob in the game, and the fact that this guy found two at the same time is crazy.