Serebii says this:
A brand new feature of the Pokémon X & Y games are Mega Evolutions. These are special features of Pokémon, different to evolutions and forms, that have your Pokémon Mega Evolve in battle into these appearances. Their abilities, stats and sometimes even types are different. To get a Pokémon to Mega Evolve, they need to be holding a special Mega Stone item unique to their species, as well as receiving the Mega Ring and the Key Stone. To get your Pokémon to Mega Evolve, you can select a button when picking the move and then select a move. The Pokémon will both evolve and attack in that turn. You may only have one Mega Evolution in a battle.
So, it sounds like each pokemon has a specific item that allows them to do it. Blazikenite and Lucarionite are supposedly already confirmed. So we'll probably have Charizardite and Venusaurite, etc. for it too.
Honestly, it seems unneeded still and pretty gamebreaking in a way if you evolve and still attack on the same turn... Most of the pokemon that don't get Mega Evolutions and weren't that great before will be even more at a disadvantage now.


