Beholders
Owlbear
Carrion Crawler
Sahuagin
Gelatinous Cube
Morkoth
Ethergaunt
Decapus, Flumph, Gambado, Lurker Above, Piercer, and many other übergoofy monsters
Disenchanter
Thought-Eater (especially its original, platypus-looking, version)
Brain Mole
I remember having read an excerpt of a pulp that was posted on a previous avatar of these boards as a possible origin of the mind-flayer. Yuan-ti are also very "pulpish".
That said, beholders are now hardly "unique to D&D"... The concept has been copied so many times (especially in computer games: Duke Nukem's Octobrains, Might & Magic's Evil Eyes (who seems to have been crossed with grells, by the way), Ultima's Gazer, and not to forget Doom's Cacodemon and Pain Elemental (although those were more ripped off from the head of the astral dreadnought) are all big floating heads with a huge eye and, more often than not, tentacles.