Nintendo released another announcement trailer for X and Y this morning. This one details the evolutions of your starter Pokemon. It also shows mega evolution variations for Mewtwo, with incredible naming creativity they are Mewto X and Mewtwo Y. The video also let's us take a look at Mega Garchomp, which is as awesome as it sounds. You can see the photos below of the starters and their evolutions. The starter is on the left and its evo is on the right.
Top: Chespin, Bottom: Quilladin
Top: Fennekin Bottom: Braixen
Top: Froakie Bottom: Frogadier
Mega Mewtwo X
Mega Mewtwo Y