A short video from Nintendo confirms rumors following the reveal of a new legendary Pokémon for Pokémon X and Pokémon Y: Yes, this new legendary is related to classic first-generation legendary Mewtwo.
The video is brief, showing an animated scene in which Mewtwo disappears in a pillar of blue light, leaving the newcomer in its place.
Mewtwo was the final Pokémon many gamers encountered in the original Pokémon Red & Pokémon Blue, lying in wait at the bottom of a bonus dungeon only accessible after beating the game proper. In-universe, it was the clone of the legendary Mew, the 151st Pokémon that players could only obtain at Nintendo events, or by trading.
Pokémon X and Pokémon Y are scheduled for release on 3DS this year.