Names to Run Away From Really Fast/Noun X

A form of Names to Run Away From Really Fast: Some people end up going by a pseudonym where they have a one-word description followed by an X. It's generally not a good idea to press them for real names.

Comic Books

 * Marvel Comics, who publish X-Men, has Professor X, Weapon X, Agent X, and Mutant X.
 * X-23 is an Opposite Gender Clone of Wolverine (aka "Weapon X") who has been trained as an assassin since basically birth.

Video Games

 * While Mega Man X himself is a heavy subversion in the games proper, he does have an evil counterpart from the Megamission card game, "Mega Man iX". And then there's Copy-X to consider...
 * Longshoreman X (yes, really) is a boss in the original Dead to Rights. He fights with full fisherman gear and a spear gun.

Web Original

 * Gaia Online has Labtech X.

Western Animation

 * Red X; not just the only antihero in the series but also the only character to never be defeated! (from the animated Teen Titans, with a color to boot!)
 * Beast Wars gives us Protoform X, complete with a big ugly X on his stasis pod. Becomes the aptly named Predacon Rampage once he's released.

Real Life

 * Malcolm X, why not. Real life Badass Preacher extraordinaire, not to mention (at least from his own perspective) an epic Heel Face Turn when he left the black separatist Nation of Islam for mainstream Islam.
 * There is an as of yet unidentified pliosaur known as Predator X. Estimates put its length at about 50 feet and give it a bite force more powerful then a Tyrannosaurus Rex.
 * Planet X
 * Angelspit's Destroy X.