Playgrounders, I beseech your great expertise in all things D&D to help me find out answers to the question posed in topic title. Some clarifications first, though:
- a D&D unoriginal monster would be one that has real-world mythological precedent and isn't sufficiently removed from it by modifications and D&Disms added to it by various D&D authors
- or one that has similar widespread fictional precedent
So, 1) would disqualify vampires, werewolves, faeries, zombies, angels, demons, devils etc.
and 2) would disqualify orcs, dragons, Tolkien-style elves and dwarves etc.
Well, some would fit both, as the line between 1 and 2 isn't always clear, but it is of no matter to this question.
Also, I'm mostly interested in richly described and innately diverse D&D original monsters - ones that could serve as a basic for a whole campaign, instead of a single encounter or - at most - 1 adventure. For example aurumvorax AFAIK is quite original to D&D, but for how long could you make a gold-eating, six-legged weasel interesting? Not very long, not by itself.
However, things like Drow, Illithids and Beholders certainly qualify. What else?