Neothelid

Neothelids are ginormous worm-like aberrations.

Neos differ greatly between D&D and Pathfinder. In D&D, neos are overgrown illithid tadpoles, and are mindless. On Golarion, however, they are the highly intelligent children of the Outer God Shub-Niggurath.

D&D

Long story short, mind flayers reproduce by basically nutting in a big pool of brine and then taking their tadpoles and inserting them into your skull. A lot of uncomfortable details later, and boom you have a mind flayer.

Neothelids are what happen when illithid tadpoles are left in said pool of brine and are never inserted into a human. Basically, they get bigger, and the big ones eat their smaller siblings until only one tadpole is left. This tadpole then grows. And grows. And grows. Until—boom—you've got the giant worm thing on the top right of your screen.

These things are nasty and are an illithid colony's worst nightmare, namely because neothelids can't be controlled psychically. They lack the intelligence for that.

Yes, you heard it here first: neothelids are literally so fucking stupid that mind flayers can't kill them.