The official Pokemon website has been updated this morning, revealing the names of the two legendary Pokemon revealed in the trailer for Pokemon X and Pokemon Y. The blue one is named Xerneas(ZURR-nee-us)while the red one is named Yveltal(ee-VELL-tall). These names are confirmed for international languages as well.
As if it weren't already obvious before this reveal, the names help confirm that Xerneas is most likely the legendary Pokemon for Pokemon X, while Yveltal is most likely the legendary Pokemon for Pokemon Y.
Xerneas (pronounced ZURR-nee-us) is the mascot Legendary Pokémon for Pokémon X and is thus found on the box for Pokémon X. Its counterpart is Yveltal, who is the mascot for Pokémon Y and is found on that game's box instead.
It was first revealed on January 8th, 2013 during the grand reveal of Pokémon X & Y.