Anonster is one of the bosses that Wario faces in the game Wario Land 3. It can be found in Out of the Woods, inside of a gargantuan tree. Anonster is the guardian of the last Music Box.
Anonster is a colossal spider with three eyes and a circular "target" mark on its back that changes color each time it's hit. When Wario reaches Anonster, it's connected to the roof via its spider-web. Anonster spews a web string that can turn him into Ball o' String Wario and kick him out of the battlefield.
To defeat it, Wario has to wait until the web projectile touches the ground and turns into a ball. He must grope it and throw it at Anonster to make it fall. When Anonster does, it'll shoot web projectiles at Wario. He has to Ground Pound above to damage Anonster. When done so, the spider will return to the ceiling and repeat its strategy. After receiving enough hits, Anonster will be defeated and Wario can access the final music box.