Nintendo is mixing up the starters formula in Pokémon X & Y.
Not only will you be able to choose from the three newcomers, Chespin, Fennekin and Froakie, Professor Sycamore will additionally offer you one of the classic starters from Pokémon Red and Blue -- Bulbasaur, Charmander and Squirtle.
The three retro Pokémon will evolve into their final forms, Venusaur, Charizard and Blastoise, and once you have a Mega Stone they will be able to Mega Evolve while in battle.
Nintendo also revealed that Mega Evolutions aren't that easy. As well as the Mega Stone, trainers will need to have the Mega Ring that holds a Key Stone to trigger the otherwise impossible transformation.