Martense Kin

The Martense family, through generations of inbreeding, degenerated into animalistic monsters. They left the surface world to dwell underground in labyrinthine warrens, shunning the light—though it doesn't actually hurt them.

In the RPG...

Martense Kin used to have human stats except for lower SIZ and INT. The 7e Malleus Monstrorum has changed this, however. They still have the same averages, just different ranges. It's said that "swarms of hundreds of these creatures are not uncommon", which would be their biggest threat. Even the smallest group is said to be at least a dozen. They are presumably only active in the Catskills mountains, however, so any encounter would have to involve that location in some way, making them a rare enemy, though similarly devolved humans could be found elsewhere.

In a video game...

With their numbers, Martense Kin would make for suitable cannon fodder, but with the limitation on their habitat, I think Rat People—or even Ghouls—would be better utilised.

In a film...

There have been a few adaptations of Lovecraft's The Lurking Fear—which the Martense Kin are lifted from—but none have been particularly good or scary, which is a shame, because I believe the story could make for a great horror film.