i think Xerneas and Yveltal are definitely based off of Norse MYthology. Xerneas is based off of the four stags surrounding the tree of life, also each of the stags is said to have four different colored gemstones in their horns, the colors are blue, purple, red, and gold, the exact same colors that Xerneas has in his horns.
then Yveltal is based off of the blind eagle at the top of Yggdrasil, also yveltal has blue tinted eyes hinting at the blindness of the eagle.
then everyone thinks there will be a Z pokemon, well in norse mythology there is also a serpentine creature, the serpentine dragon Niohoggr, he is lying under the roots of the tree of life. and a serpent is the only creature really able to make a real Z shape. and also if there was a legendary pokemon based off of Niohoggr it would be the first Dragon/poison type