Charizard's second mega evolution was shown during the Japanese broadcast of Pokemon Origins. The first Mega Evolution, which was shown a few weeks ago, seems exclusive to Pokemon Y.
In X and Y Charizard can Mega Evolve into either Mega Charizard X or Mega Charizard Y, depending on which "Mega Stone" you give it. Charizardite X will make your Charizard's body turn black and its flames blue. It will also change from a fire and flying-type Pokémon to fire and dragon-type.
Charizardite Y will turn your Charizard into 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.
Pokemon X&Y hits Nintendo 3DS on October 12.