Mega Charizard X, making its big debut on October 12 in the two new Pokemon titles, evolves temporarily via careful application of Charizardite X, only available in Pokemon X. Where Mega Charizard Y's evolution greatly increases its SP attack, X gets a straight-up attack boost. Y's ability becomes Drought and it learns the powerful Blast Burn skill. X's ability is Tough Claws and its skill is Dragon Tail, hitting enemies so hard they switch out in battle.
The biggest difference, however, is that coloring. It's like they flipped him in Photoshop.
He's so blue.