Neh-thalgu - otherwise known as a Brain Eater - they have a disgusting, huge updated version in the Epic book, but I remember the first appearance in the Expert module, Castle Amber - a strange cross between an spongy arachnid/lobster body, multi-eyed eel head with a tentacle fringe and a lamprey like mouth. It was enormously intelligent, and it came from it's dimension in search of spell power - by extracting brains - not for taste and preference like a mind flayer - but as a magical capacitor. It would crack open a helpless person's skull and swallow the brain and all its fluids whole - but instead of digesting it - it would be ferried to an empty pimple like sac on it's neck - there the brain, its fluids and all it's memories and knowledge would be stored for the Thalgu to access - and in the event it acquired a mage's brain, it could revitalize it daily, and unleash one or more spells from the brain. And those sacs were translucent - so you could see the extra brains just under it's slimy skin - and even see them arrive shortly after ingestion. And then, the most disturbing thought is - do the brains still have the thoughts of the original owner running in them - do they realize they've been taken from their own bodies - does the thalgu taunt them and torture them in their own mind ...yikes.
In my mind, that puts the old Illithids to shame....
That would freak me out...
The One Warlock