Mewtwo

Mewtwo is a Psychic-type Pokémon. It is a clone of Mew and is one of the types of Pokémon.

Pokémon Games
Mewtwo appeared as a one-chance battle in the Cerulean Cave (previously known as the Unknown Dungeon) in the Red, Blue, Yellow, FireRed, & LeafGreen versions, accessible after defeating the Elite Four and League Champion. He has also appeared in side-series Pokémon titles, as well as two feature films.

In the Nintendo 64 game, Pokémon Stadium, he was the final battle challenge after defeating the Gym Leader Castle and getting all of the trophies in the Stadium. Defeating him rolls the credits, changes the title screen image, and unlocks Round 2 of the Stadium. Defeating Mewtwo in Round 2 is the final test in the game (besides getting all 151 Pokémon into the Hall of Fame).

Mewtwo can be transferred into other Pokémon games. From Red/Blue/Yellow: Gold, Silver, Crystal, Stadium, Stadium 2 From FireRed/LeafGreen: Ruby, Sapphire, Emerald, Collosseum, XD: Gale of Darkness, Diamond/Pearl (permanent transfer), Battle Revolution (from Diamond/Pearl).

Mewtwo was also featured as a playable character in Super Smash Bros. Melee.

Super Smash Bros.
Mewtwo appeared as an unlockable character in Melee, available to challenge after 20+ hours of VS. matches or 700+ VS. matches. Ironically, Mewtwo is considered one of the worst characters in the game, with recent rankings putting it ahead of only Kirby and Pichu. Mewtwo is not playable in Brawl, but Lucario, a new playable Pokémon, has a similar play-style as Mewtwo. Mewtwo also appears as a trophy, which can be unlocked by playing All-Star Mode on Intense difficulty.

Trivia

 * Despite being a clone of Mew, it comes first of the two in the Pokédex.