In Nintendo’s last Nintendo Direct, called Pokemon Direct, they described the strange starting conditions for Pokemon X and Y where you get to choose not one, but two starter Pokemon. Your best friend (who you get to name) gives you one of the X and Y generation starters in the form of Chespin, Fennekin, or Froakie. However, when you later visit the Pokemon Professor to get your Pokedex and Pokegear, he also lets you choose another starting Pokemon from a selection of Bulbasaur, Charmander, or Squirtle.
Yes, you will get one generation-six starter and one generation-one starter! Not only that, but Nintendo is introducing brand-new evolution levels to the game called “Mega Evolutions.” Mega Evolutions allow Pokemon to evolve beyond their most evolved state. This happens when “the mega ring that a trainer wears reacts with the mega stone a Pokemon holds.” It seems to have something to do with Pokemon happiness or friendship levels as well.
Unfortunately, it appears as if this can only happen in battle, or at the very least, no video was shown of these evolutions being permanent. Venusaur, Charizard, and Blastoise are only some of the Pokemon that have Mega Evolutions. The trailer later shows Mawile, Lucario, Absol, Ampharos, and even Mewtwo to have Mega Evolutions as well. Yes, the Mewthree image that everyone has been going nuts about is Mewtwo’s Mega Evolution.
Source: Pokemon Direct