Wart
Wart is the main antagonist of Super Mario Bros. 2, although Wart made his first actual appearance in the game's original counterpart, Yume Kōjō: Doki Doki Panic. He is also a character of The Legend of Zelda franchise, appearing in The Legend of Zelda: Link's Awakening. Wart has seldomly appeared in official media, but he has appeared considerably more often in fanon media.
Fanon appearances
Neglected Mario Characters
Since Wart has only ever appeared in one Mario game, he is one of the primary guests in the web comic, Neglected Mario Characters. He is often portrayed as a hitman for Bowser. Unfortunately, this gets him into a lot of trouble. For example, Bowser often sends him into the middle of an NC Deathmatch to do his dirty work while he is busy fighting off the Mario Bros. He joined Bowser's Dual Alliance of Death (DAD) consisting of Bowser and Luigi to make it a Triple Alliance of Death to fight off Mario and the Koopa Kids. He was used to lure the Koopa Kids into a trap, but failed. A recurring joke on the site is to portray him as a rambling old man who hates children, and thus there are many times when he gets into brawls with the Koopalings, usually Lemmy Koopa.
Rise of the Mushroom Kingdom
Wart is the main antagonist of the Rise of the Mushroom Kingdom series. There, he attempts to kill Mario, but ends up murdering Luigi instead, sending Mario on a quest for vengeance.
Fan theories
Wart is said to be related to Bowser — perhaps due to the fact that some of Wart's own minions have been shown to be working for Bowser in later games. Some fans claim he is Bowser's older brother who was refused the Koopa Kingdom because he was turned into a frog due to a magical accident (which Bowser may be responsible for). Like many seemingly forgotten Mario characters, Wart is a popular villain in fan fiction.