My name doesn't matter. I never keep the same one for more than a few days.
~ Maya Brooks
If he had a Name, it's long since been lost.
~ Gandalf on the Witch-King of Angmar

Villains whose real, given names are unknown (if they had one to begin with, EX: The Thing), simply forgotten (EX: Cheryl Tunt), are unpronounceable by human language (EX: Glgrdsklechhh), and/or are known only by aliases or conjectural names (EX: Maya Brooks).