Charizard becomes part-dragon as a Pokemon X exclusive.
Mewtwo isn't the only one with mega evolutions exclusive to each version of Pokemon X and Y. Today the Pokemon Company revealed that the original evolved fire starter has more than just the one mega evolution we've seen so far. In Pokemon X, Charizard can evolve into Mega Charizard X. Instead of its fire/flying-type version in Pokemon Y, Mega Charizard X changes type to fire/dragon and its appearance becomes black and blue. Mega evolutions only occur in-battle, so the difference between the two different Mega Charizard will not be in effect at all times.
The Pokemon Company disclosed the news on its website and in a YouTube video that depicted a sky battle. Flying Pokemon and those with the ability "levitate" can fight in the air. It appears that certain other Pokemon can participate as well, as in the case of part-dragon-type Mega Charizard X, maybe because it can learn the move "fly." Or maybe it's because Charizard is part-flying and will enter the sky battle before it can mega evolve.
Mega Charizard X gains the ability "tough claws," which raises the power of direct contact moves, and its attack increases significantly - as opposed to Mega Charizard Y's increased special attack and fire-type moves.
Likely, we will see different mega evolutions for both Blastoise and Venosaur. Hopefully something new will come out for those of us who decided to go with Pokemon Y.