Zidhurot.Z+ Nidhug (The nordic snake,written in danish.)+ Rot (Root in nordic)
So this was my initial take at the Third Legendary, for a supposed "Pokemon Z" game.
You may wanna take a look at this, to understand the whole idea behind this snake: [link]
Its quite interesting.
So, supposing that Yveltal and Xerneas are Based on Norse mithology, the third legendary would be based on "Nidhogg" The snake who gnaws at the roots of the Yggdrasill World Tree. [link]
(And that's why I added the jaw appendix below its head.)
And here is something even MORE interesting, taken from Wikipedia:
"The squirrel called Ratatöskr runs up and down the length of the Ash, bearing envious words between the eagle and Nídhöggr [the snake]."
So there seems to be a fourth being (A squirrel?) hidden in all of this, that could bring a very dangerous conflict between Yveltal and "Nidhoggr"
This "squirrel" could either be:
A fourth, minor legendary (Much like the "Mew" of the game.)
Or the main villain, who could release the snake legendary, to fight against Yveltal, with cataclismic consequences.
Who knows? We probably must wait till "Pokemon Z" to find out.