CoroCoro's now out, and the only thing of note in it is that it confirms the new Mewtwo form is indeed just that. It doesn't outright refer to it as the Éclair form however, but at this point there's no reason to believe it's a fake name.
I'm a bit surprised at how many people are shocked by this. Despite the previous marketing on Pokémon Smash and the official sites, it hardly looked like a brand new Pokémon anyway. I blame Serebii for putting that info not on the front page where it belongs, but in a remote corner of his site where no one would ever think to look.
Now, here's an existential question: do you consider this a gen 1 Pokémon, since the species itself was introduced in gen 1, or a gen 6 Pokémon since that's when it's first going to appear? After all, this is the first time a Pokémon gets a new form in a generation that's not its own. And as far as I can tell, should Mewtwo be obtainable in X/Y, it's going to be the first time a previous legendary is available in the first pair of games of a generation.