What Nintendo has done with Charizard is amazing!
Mega Charizard X
Charizard can become Charizard X by holding a Charizadite X. He can then Mega Evolve into Mega Charizard X, and he does not just get a power boost–he changes completely! He changes from a flying and fire type to a dragon and fire type Pokémon! He changes from the beloved orange to an intimidating black and his flames turn blue with increased heat!
Mega Charizard Y
Similar to Mega Charizard X, you can get Mega Charizard Y by giving your Charizard a Charizardite Y. Once Charizard Mega Evolves to Mega Charizard Y, it has more pointed horns and its flying ability increases because of its increased wing size!
I would be careful though, you can only get Mega Charizard X in Pokémon X and you can only get Mega Charizard Y in Pokémon Y! So choose your game wisely (or buy both)!