Mewtwo was the ultimate Pokemon to get in the original Red and Blue games, but he was left out of the original Smash Bros. Mewtwo did appear in it's sequel Super Smash Bros. Melee.
I really liked Mewtwo, even though the big shadow ball here is a big copy of Samus' main attack. But when it's the best move, who can complain? I also liked Mewtwo's physical strength and the ability to dodge with the teleport move.
It's a shame that Mewtwo was kicked out of Brawl in favor of Lucario. I always thought that was a bit of a bad move. I doubt Mewtwo will appear in the 4th game set to be on WiiU and 3DS.