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.
Displacer beast are one of my favourite D&D original monsters, thri-kriin are also quite cool.
AFAIK the Tarrasque is a french myth or something like that.
Edit: Yep according to wikipedia, the Tarrasque is based on the French myth of the Tarasque