Do not ask why the period in the title is there; it's for weird editing reasons.

Pokémon: The First Movie - Mewtwo Strikes Back was an animated feature film in the Pokémon series released in 1998. It's one of the extremely few non-video game sources that are referenced in Super Smash Bros. Clash, albeit it's not a lot. Obviously as the title suggests, this was the first feature-length film in the Pokémon series. It focuses on Mewtwo, a modified clone of Mew created by Team Rocket scientists. Mewtwo later goes berserk and kills them all. Later, he travels to New Island and challenges trainers to take him on and wonders what humans really are worth.

He gets better, though.

Represntation in Super Smash Bros. ClashEdit

The Mewtwo in Clash is this very specific Mewtwo who was an antagonist in the film. Even though Mewtwo is listed as a villain in The Dark Tournament, Super Smash Bros. Clash takes place after this film, where he is now a good guy and fights as more of an antivillain. A few of his attacks, namely his Standard Special, Shadow Ball (which, at the time of this film, was not actually an attack in the Pokémon games since Pokémon Gold & Silver had not yet been released) originate from this film.

Mewtwo's alternate costume, where he dons a suit of armor, is also taken from this film's "Origin of Mewtwo" segment. When he was still an experiment under Team Rocket, he wore this armor and even broke out of the laboratory donning it. This suit of armor (and presumably Mewtwo himself) also appeared in an episode of the Pokémon anime where Gary fought Giovanni at the Viridian City Gym.