From the Super Mario Wiki
The Wario Impostor is the seventh boss in the NES version of the game Wario's Woods. He resembles Wario and has the power to make one whole row of mixed enemies fall. After defeating the boss, Wario pretends to admit defeat, but quickly informs Toad that the Wario he defeated was a fake one, and the real one is "ready to rumble!" Toad must then clear thirty more rounds and defeat Boom and Seizer again before fighting the "real" Wario.