Someone mentioned Xerneas' horns looking like chromosomes and Yveltal's markings looking like nerves.
To me it looks more like Yveltal's patterns are blood vessels. Notice in the trailer how he flies in front of the sun and the patterns shine red, just like holeding your hand up to a light bulb.
Furthermore, i'd say that Xerneas' horns are supposed to be nerves. This would explain the colorful horns. When nerves fire the light up with electricity
BW FC: 3181 5391 1618