The most menacing Pokémon of all is back.
EDIT: to quote Nintendo on this, so there's no confusion. "This series veteran fighter will be made available as free downloadable content in spring of 2015 for anyone who buys both versions of the game."
Apparently they're not done building him yet! Here is an in-progress GIF:
Complete speculation, but I'm guessing he won't be the only downloadable character we'll see in the future. And I gotta give props to Nintendo for giving people incentive to purchase both versions of the game, too. Clever, clever.
We contacted Nintendo to ask if Mewtwo will be available for players who only have one version of the game, and will update this once we hear back.