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.