Back in 1981, my mother bought me a set of 5 AD&D books: the Players' Handbook, Dungeon Master's Guide, Monster Manual, Deities and Demigods and the Fiend Folio. Despite being very uneven in content, the Fiend Folio left an indelible mark on me, presenting several odd and creepy monsters whose main attraction was their weird "what the hell was that?" factor.
One of my personal favorites was always the grell. Why? Well, it was a levitating brain with a crapload of paralyzing tentacles, a big beak, and an inexplicable resistance to electrical attacks. I mean, look at that picture. That's the stuff of I-ate-pepperoni-pizza-just-before-bedtime nightmares. There's no point to a giant brain, and no way to figure out what the hell it's doing here. It's a deadly, freaky death-brain, man. And it attacks you 11 times a round, paralyzes you, and then rips the hell out of you the next round. What is there to explain?