Pokemon Smash! just aired and with it we we're officially revealed what appears to be a new Pokemon related in some way to Mewtwo!
Nothing officially relates it to Mewtwo at this time.
We should get new localized info within the hour, so I'll update this post once we know more!
Photo courtesy of Serebii.net
UPDATE: Pokemon.com has officially updated with a feature about this new mysterious Pokemon. Hinting at obvious ties to Mewtwo, but not officially relaying any connections.
Nice selection of pics, and a Video for your viewing pleasure.
My guess is that there is some ties to the speculated "Bonds" system here. Sylveons typing still hasn't been announced, but I'm betting that these two are both related to that in some way, with Sylveon being a bond evolution of Eevee.