I'm doing my X Nuzlocke run and for kicks I decided to write down everything the man in Anistar City says about the myth of Xerneas.
>It happened about 800 years ago. The Pokémon known as Xerneas used it's shining horns to illuminate the lands of Kalos. At that very moment, people and Pokémon throughout the land felt great energy and vitality surge through their bodies. At the same time, a vast forest sprang up, with Xerneas at its center. Legend has it that, when it nears the end of its thousand-year lifespan, Xerneas releases its remaining energy, sharing it with all living things nearby. There's another story I know that talks about the appearance of just such a fortuitous Pokémon.
>Back 3,000 years ago, the Kalos region was engulfed in a terrible war. So the tale goes, a Pokémon came to the rescue of all the wounded Pokémon that had been caught up in the conflict. Some people expected the Pokémon that appeared was Xerneas, but there's no way to know. Maybe they just wished a Legendary Pokémon would come to brighten a tale of woe.
>Supposedly, after Xerneas released its remaining energy, it transformed into a dried-up tree. It remains in hiding deep within the forest. According to old tales, I say. Stories and tales.
But what does he say about Yveltal, in Pokémon Y?