Actually, it seems to me like Yveltal is based off of a wyvern, which is a type of dragon with only two legs instead of four. Although it is a very difficult decision, because I love both of them, I'd have to say that Xerneas is my favorite. It's so beautiful, and it reminds me of the spirit of the forest from Princess Mononoke. If they do make a Z, I can't imagine what it would be. They seem to be doing an amazing job on the new Pokémon so far.