Sisyphus is likely named after the species of dung beetle (a.k.a. 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".