Yveltal side arms are apparently wing's =''YVELTAL'' Bulbapedia=
veltal is a large avian-like Pokémon. Its body is dark in coloration and is adorned with grayish patterns along the underside. Yveltal's wings and tails have three points extending along the edges, close to where they meet the body. There are five large claws on each appendage, three of which curve inwards. Yveltal's underside is bright red, with branching, black markings. Similar markings are present on Yveltal's head and neck.
Source = information from site.