It's over, guys.
Mewtwo's new form confirmed.

This guy right here posting on a Spanish forum is Hiro, who works with Melkor in Pokéxperto. He is saying this is not a fake.

You may not know who they are (because you probably don't know Spanish and therefore don't frequent Spanish pokémon sites) but these guys have always been correct on previous information and rumours, Hiro (here as F. Lombardi) works for GameFreak in an unspecified position.

Maybe you will remember they announced Black and White before 5th gen was a thing, confirmed the starter evolutions and described all 5th gen pokémon before they were leaked for the first time. But you probably don't remember because you only trust serebii or some other anglo guy.

Believe what you want. But considering that Pokémon Sunday will air soon, we will probably have an official confirmation.

tl;dr new form confirmed