As a bird based Pokémon,it has huge flaps that are red colored internally and black colored externally.When Yveltal straightens its arms in flight, it forms the shape of the letter "Y". and that's why the game is called pokémon y
Yveltal represents destruction, so it seems to be an angry Pokémon.
Its form may be a reference to Norse mythology guardians of the Yggdrasil tree: a bird-like creature that protects the top of the tree, this may be Yveltal, a deer-like creature that protects the soil and trunk of the tree, this may be Xerneas, and a snake-like creature that protects the underworld below the tree is Zygarde.
Its name may come from the word tall since it can fly and start with the letter Y because it represents the Y axis in the 3D axis of X, Y and Z.