They seemingly are the only permanent residents of this place, and are similar in look to cockroaches if you have those on your world.

[He can't look for them like she is, but Ignis does listen for them, and for the moment at least, he hears nothing.]

The big difference is they have the ability to increase the fear you're feeling. I've felt it myself and I suspect if they increase the fear far enough, they may make you see things that aren't actually there.