Originally Posted by Edge of Dreams
I've always been fond of Displacer Beasts and Blink Dogs. No clue if those have any mythological background, but I don't think I've encountered them in any other setting.
The Tarrasque is also pretty iconic of D&D, I think.
As for intelligent races that are notable in D&D, I'd like to add Yuan-Ti and Elan to the list.
I agree with the above, although I'm not sure the Tarrasque is completely iconic, since everyone I know says "You mean that thing that's essentially Godzilla?" whenever I bring it up.