D&D Monsters: Mind Flayers

The mind flayer is a creature original to D&D and, like many of Gygax's early creations has a "does what it says on the tin" style name. To my mind, it's one of the better such names - although almost anything is better than, say, "lurker above". The look of the creature is obviously reminiscent of H.P. Lovecraft's Cthulhu, and Gygax stated that it was inspired by the cover of a book by Brian Lumley that does, indeed, feature a being from the Mythos. But, as it happens, it isn't the Big C himself that's on the cover, but a chthonian, a creature with a similar name, to be sure, but actually rather different. Still, it has tentacles and this, seemingly, was enough.

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1E

The mind flayer goes through relatively little change in physical appearance through the various editions of the game, to the extent that the few changes that are apparent could easily be put down to variations between individuals of the same species. Even the clothing doesn't change much, with the race clearly having a preference for robes and insisting on a skull motif somewhere on their belt. 

D&D Monsters: Umber Hulks

The origins of the D&D umber hulk are not clear. It predates what we'd now call 1E, first appearing in the Greyhawk supplement, and it has been proposed that it may be based on one of the same set of plastic toys that inspired the owlbear, rust monster, and bulette. If so, it doesn't closely resemble any of them, while the other three are very clearly drawn to resemble the toys in the 1E Monster Manual. So, if it's inspired by them at all, the connection is still vague enough that, to all intents and purposes, it's an original creation. Certainly, its signature power is original, doubtless intended to be related to its unusual appearance and, while evil sorcerers might be able to do something similar in myth, the same is not generally true of monsters.