Re: Best D&D original monsters out there?
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 spent an hour on the edge of dreams,
I walked between the worlds,
and when I woke I never knew
to which side I had fallen