Another candidate would be Githyanki. I don't think there is anything like them anywhere else.
And actually, their backstory is as stupid as the worst out there: A race of space pirates that live in a void without time and build their cities on the petrified bodies of dead gods. And their queen is a necromancer who kills everyone who could become stong enough to threaten her rule. And they have Red Dragons as pets.
But somehow, they are still cool!?
I thought your description of the Githyanki was cool right up until the part where you said they keep red dragons as pets (Because really, it should be the other way round.)
and as for a unique D&D invented creature, the humble mimic, and the huge variety of similar beings that disguise themselves as objects deserve a mention.