My vote would be for the Bulette which I don't believe has any mythological roots. The Cat Lord, the Beholder and the Illithid are a few others... and for some strange reason I've always had a soft spot for the old D&D "dungeon monster" standby's - the Gelatinous Cube, Green Slime, Piercers and so on.
Don't ask me why...
Cheers,
A'koss.