Complex sentence: Mewtwo is now able to transform into a new form called in the films, but later revealed to be Mewtwo's form in the Mega Evolution mechanic found in "Pokémon X" and "Pokémon Y".
Split into simpler sentences: Mewtwo is now able to transform into a new form called. This is later revealed to be "Mega Mewtwo" in the new Mega Evolution mechanic for "Pokémon X" and "Pokémon Y".