In Pokemon X and Y
In the trailer, Xerneas's eye has an X in it, and Yveltal is shaped like a Y, meaning they will most likely be obtained in the game that corresponds with their letter.
Update: This has now been confirmed - Xerneas (the deer) will appear in Pokemon X. And Yveltal in Pokemon Y of course.
You will get the deer like legendary Pokemon in Pokemon X version. His name is Xerneas.Pronunciation(X)(er)(ne)(as)
In Pokemon Y version you will get Yveltal, the bird-like legendary Pokemon.Pronunciation(Y)(vel)(tal) Source* https://www.facebook.com/Pokemon?ref=stream