Aboleth are aquatic aberrations with impressive psionic abilities (or magic, depending on edition). Can Dominate, as well as force you to lose the ability to breath air. Creators of the Skum, descendants of humanoid slaves that are enthralled to the aboleth. Also, their species is older than the world and the gods themselves, and they can back up that claim thanks to their ancestral memory, meaning that they are incapable of forgetting and all their memories are passed down from parent to offspring. They can also absorb the memories of anything they eat. The only thing they don't recall is the genesis of the mind flayer species (since they're from the distant future) so of course this freaks the aboleths out a little bit since they have no idea where the fuck they came from.
They first appeared in the Dwellers of the Forbidden City adventure module in 1981, and then in Monster Manual II a few years later. Dragon Magazine #131 has an article called Ecology of the Aboleth. The normal aboleth are merely well... normal. There are larger and more powerful aboleth. With psychic abilities that would put The Emperor and Professor X to shame. Mother Brain aint got SHIT on the most powerful one. Sadly this article has seemed to have been ignored by everything that came after it, even though 2nd edition had some fantastic campaign and lore books for a lot of races.