Campaign Lore

A powerful psychic fish monster

...an ancient race of fish-like amphibians, usually found lurking in subterranean waters. One resembles a huge, slimy fish, with three large eyes and four long, sticky tentacles arranged around its mouth. An aboleth secretes an oily, foul-smelling slime, polluting the water where the creature lurks.

Sources

The Basic Fantasy Field Guide, Volume 1