The creatures first appeared in H. P. Lovecraft's short story "The Shadow out of Time" (1936). He never gave an official name for the monster, though, they're often called Elder Beings or Polypous Race in English and 盲目のもの (Moumoku no Mono, lit. Blind Ones) in Japanese.
The Great Race of Yith gained a temporary victory over them using their energy weapons. Yithians drove them underground and sealed the tunnels to the surface. Eventually, the Polyps rose up and got their revenge on the Great Race.
They're sometimes depicted as floating amorphic monstruosities with tentacles.
