I posted my theory in another thread, and even though it was 'out there', I think Noivern helps my case a bit. It's still very out there, but just hear me out. In one of my posts, I suggested that Xerneas and Yveltal might be 'altered' Pokémon. I suggested that Xerneas was an altered Sawsbuck, and Yveltal was an altered unreleased Pokémon. With the unveiling of Noivern, it makes me think that maybe Yveltal is an altered Noivern. Both Yveltal and Noivern have those black claw-like things, and a white tuft around their neck. That's my crazy theory.
Vivillon is amazing. It's so mesmerizingly beautiful and I want one now!