Mewtwo says that they haven't seen his true power. Probably means Mewtwo had this forme all along and just hiding it.
The truth was, he was jealous Jessie's beautiful hair from team rocket. But since he can't grow hair, he decided to rip off his tail and super glue it to his head. He then runs around calling himself Jessietwo. --- This is a sig. Yup.
Keldeo got a new form which was completely unexplained in both the movie and the game. The excuse for the Therian forms was pretty crappy too. (IIRC, in Dream Radar, Burnett speculates that they changed form by adapting to their environment.) --- Official Zangoose of the Pokemon X board+Only member of my one-man team 3DS FC: 3609-1237-6725