And so the Mega plot thickens. According to magazine scans of Japanese magazine Corocoro published by Serebii, the Mega Mewtwo that we've seen thus far (pictured above) is actually is exclusive to Pokemon Y. There's also a Mega Mewtwo X, exclusive to Pokemon X—it's supposed to be psychic/fighting type, with the "Steadfast" ability.
You can view a picture of Mega Mewtwo X here...honestly? It doesn't look as cool as Mewtwo Y! Oh, and there's more info too.
- Garchomp, the infamous dragon Pokemon, is supposed to have a Mega evolution too. You can look at him here. If true, this is a curious choice, as Garchomp is considered to be overpowered already.
- The middle starter evolutions are now known. You can see pictures of them here, Chespin's in particular is rather cute!
- There are a few new Pokemon shown, too—and a few more details which Japanese forum-posters speak to, but there are no pictures for.
Kotaku contacted The Pokemon Company about this information, and they issued the following statement:
There's nothing to officially report at this time. Please stay tuned for when official announcements are made by The Pokémon Company International.
Which, y'know. Isn't an outright denial that this stuff is true, but as is usually the case with stuff like this, we'll just have to hang tight and see. EDIT: since some people are complaining, this is less a matter of not trusting CoroCoro, and more a matter of us always reaching out to companies before publishing rumors—recall that even Serebii says they can't confirm some info. Additionally, Nintendo is able to provide not only context, but perhaps additional assets we can show you. So yes, it's worth contacting them about.
It's worth noting that previous titles in the franchise use game-specific exclusives as a means of encouraging trading between players—so the Mewtwo stuff doesn't sound outlandish. What we don't know is if other Pokemon will also have more than one Mega evolution. However, we do know that Mega evolutions require specific stones, and stones are sometimes used to give a Pokemon multiple choices when it comes to evolution—think about Eevee, for example.
And finally, in case you're confused: Mega Evolutions are refer to temporary power-ups which Pokemon can receive during battle. Unlike some power-ups in the game, Mega Evolutions change the appearance of the Pokemon in question. You can read more specific details about Mega Evolutions here, and see the thus-far confirmed Mega Evolutions here, here and here.