So basically, the legendaries are based off Norse Mythology. I don’t know much about it, but my friend is explaining to me the tidbit information that is being used by Nintendo in designing their legendaries. Yveltal, the bird, represents the corpse eater, and thus explains his Dark type. Xerneas, the deer, represents life, alluding to its fairy type. Zygarde, the snake, is known to be attacking Yggdrasil (the realm) in Norse mythology, but it seems to differ in the Pokemon world because it is said to be monitoring the ecosystem from its cave.
I can’t wait to hear about these legendaries’ place in Kalos!