Pokémon X and Pokémon Y will launch on Nintendo 3DS in October. We don’t know tons about actual game play yet, but we do know that the pocket monsters are going 3D for the first time. We’ve also learned that there’s a Y-shaped monster called Yveltal, and an X-shaped monster (pictured above) called Xerneas. Those two creatures seem to be the mascots for each game.
One theory described on Turnip’s Tumbles explains that the X and Y in the titles refers to biology. If you look at the basic shapes of the monsters, they resemble a neuron and a blood vessel.
Xerneas and Yveltal look very organic.
Xerneas represents a Neuron, Yveltal represents Muscles and Blood.
That means 2 of the 3 germ layers are covered.
Almost all animals develop out of 3 layers.
The third missing layer is the Endoderm.
The third legendary will represent the Endoderm.
Not sure if this is accurate, but I like this line of thinking.
Check out an image mapping out the details after the break (click to enlarge).