Pokémon X & Y's Mega Charizard evolution has been discussed in a new press released published by The Pokémon Company. Check out all the details here.
The release re-iterated that Charizard can Mega Evolve into either Mega Charizard X or Mega Charizard Y, depending on which Mega Stone you give it.
Give it Charizardite X and your Charizard's body will turn black and its flames will go blue. It will also change from a fire and flying-type Pokémon to fire and dragon-type.
Give it Charizardite Y and you'll get Mega Charizard Y. In this form its special attack increases significantly, and it unlocks the ability 'Drought', which increases the power of Fire-type moves. It can also use a fire-type move called Blast Burn, which gets more powerful when you use Drought.
Pokémon X & Y hits Nintendo 3DS on October 12.