From Bulbanews, your community Pokémon newspaper.
Xerneas and Yveltal to feature in Gen VI games
The Legendary Pokémon seen but not named in the first Pokémon X and Y trailer have now been named. The deer-like Pokémon is Xerneas and the bird-like Pokémon is Yveltal.
The reveal came on the English language Pokémon X and Y site, and the official Pokémon Facebook page. In keeping with past legendaries, the names seem to be identical in French, German, Spanish and Italian. While likely to be the same in Japanese too, the katakana for the name are still unknown, making this a very rare circumstance where English names were released before the Japanese.