Mewtwo appears as the final boss after all competitions in Pokémon Stadium.
Players must complete the game’s Cups and Gym Leader Castle before unlocking a six-on-one battle against Mewtwo. Defeating it unlocks another round of Stadium, Gym Leader Castle, and the Mewtwo battle at a higher difficulty.
Mewtwo was a fitting final opponent because it is presented in the original games as an exceptionally powerful artificial Pokémon created through genetic experimentation. In Pokémon Red and Blue, the player encounters it in Cerulean Cave after defeating the Elite Four and the rival.
Mew is sometimes chosen as the answer because of its close connection to Mewtwo and its status as a rare Mythical Pokémon. However, Mew is not the final boss in the original Pokémon Stadium progression. The climactic opponent is specifically Mewtwo.