It could also be based on chromosomes! (And there isn't a Z one!)
There is no Z chromosome in the XX/XY love-determination system, which humans use.
Theres the XX/X0 system, in which there is only an X Chromosome, and nothing to match. This is often seen in insects.
There is then the ZW/ZZ system, which is like XX/XY, but in reverse. Females have two different love chromosomes, rather than the males, who in this case have two of the same chromosomes.
This is seen in birds, reptiles, and some insects and other creatures.
That doesn't really relate to what any legendary would look like, since Yveltal takes the shape of the letter of the gene unrelated to him (Y, and Yveltal appears to be some form of bird/reptile (dragon?), so wouldn't have XY genes).
But at the very least, you could have a W or Z version, or à la Black/White 2, both.
Pokémon WXY
Pokémon XYZ
Pokémon WXYZ
Take your pick.
(The second is my favourite, and probably the most logical for a third version name).
