Sisyphus is a character appearing exclusively in the MegaDrive/Genesis version of Teenage Mutant Ninja Turtles: Tournament Fighters. No back story is given to him, although he is indicated to be a friend of the Ninja Turtles. Sisyphus is a mutant scarab.
Sisyphus is likely named after the species of dung beetle (aka scarab) of the same name. Scarabs are known for rolling large balls of feces and other material, similar to the punishment given to the Greek myth character of Sisyphus.
In the Japanese version of the game, he is known as Musha Beetle. Musha is Japanese for "warrior", and it's likely also a pun on "mushi" (bug/insect) since the kana reads "むしゃ".
In the episode Outbreak of the fourth season of the Teenage Mutant Ninja Turtles (2003), there is a mutated bug that shows up in the sewers that strikes a resemblance to Sisyphus. In fact, it could be him. If so, then aside from the game Tournament Fighters on Sega Genesis, this is so far his only other appearance. Also, there are Cockroach monsters that show up and resemble the clone of him from that game.
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