Everyone Is a Tomato

"Seth Green: How about letting us come aboard and help you with your whip-smart plots?

Ron Moore: Help? Why would I need help writing plots? I just throw a dart at the cast list, and...boom! They're a Cylon! Rinse, repeat, cash the f*cking check. Watch! Sh'boom! Sh'bong! Sh'bing! Cylon. Please help me! This is so hard!"

- Robot Chicken

This trope refers to creating a fictional subcategory that can be applied to any character and then applying it ad nauseam to most of the cast. Possibly as a method to create a Meta Origin (it usually fails).

See also: Flock of Wolves. Contrast They Look Like Us Now.

Anime and Manga

 * Mahou Sensei Negima is a bit guilty of this. Although most of the cast was considered normal at the start of the series (excluding Sayo the ghost and Chachamaru the robot,) since then it has been revealed that  were all involved in magic from the beginning, and   are demonic. Asuna is practically her own category.

Comic Books

 * Lampshaded in the Great Lakes Avengers, where the team renamed themselves The Great Lakes X-Men after it was revealed that the entire team was made up of mutants.
 * This was also a parody of the then popularity of various "X-Titles".
 * It's a justified trope in the Marvelverse, mind; all the other origins are one-in-a-billion freak occurences, or inventions by 99.999999th-percentile geniuses who for some reason keep their potentially world-changing inventions to themselves. Mutants are a demographic.
 * In Strangers in Paradise, just about every  character.

Literature

 * The Man Who Was Thursday by G.K. Chesterton: The hero is drawn into the murky world of anarchists. He discovers that one of their number is an undercover policeman. As the story continues, his attempts to apprehend anarchists force all of them to reveal that they are policemen, too. The story was published in 1908; the trope is Older Than Radio, at a minimum.

Live-Action TV

 * The second Battlestar Galactica made nearly half the cast Cylons. Although there was plenty of evidence to suggest that  was one, and even that . Not to mention precedent of sleeper agents.
 * Heroes often had characters revealed to have powers.
 * Buffy the Vampire Slayer: At the start of the show, only Buffy was anything other than a normal human, but by the end, several supernatural beings had joined their group, and almost everyone else had learned how to practice magic.
 * Dollhouse: is a doll!  is a doll!  is a doll! Season 2 brings even more.
 * True Blood: The folks with powers will soon outnumber the normal people in Bon Temps.
 * V-2009: Seems like every third character is secretly a V sleeper.

Video Games

 * From Kingdom Hearts II onwards, almost every single mysterious character manages to be, no matter how illogical or unlikely it seems. The only ones who aren't are the ones who actually turn out to be

Web Comics

 * In Its Walky, several cast members are revealed to be former abductees.