The Thought Eater
From Dungeons and Dragons: Monster Manual I, the Thought Eater was a ridiculous creature of unknown inspiration.
“… dwellers in the ether … appears to be something like an enormous headed platypus”
Invisible in the physical plane, it was only seen and observed with psionic powers. (With every gift comes a price, I guess) If you happen to visit the Ethereal plane, the Thought Eater may pay a visit to eat your brains, figuratively, feeding off your thoughts and intelligence. As described in the manual, it may render you “a low-grade moron” permanently.
The Thought Eater made exactly zero appearances in my adventures. Swimming in another plane was extraordinary rare. Sending in a floating mind eating platypus was never considered. The next time your DM presents you a large, seemingly empty jar in a mad scientist’s laboratory somewhere, think twice before opening and sticking your head inside. It just might end up an idiotic decision.
Trivia: Appearing in groups between one and three, mental defenses, including a mind blank spell thwart attacks of these horrid things.