Again, as far as we know, it doesn't exist. It doesn't even make sense for there to be a Z legendary, if you think about it. Xerneas represents life and Yveltal represents death and destruction; what could a counterpart to those possibly be?
Rebirth? Zombies? Crack addicted chipmunks? It doesn't necessarily have to make 100% sense, look at 4th Gen
TIME
SPACE
ANTI-MATTER wait, what?
It kinda sort makes sense, but if you were told there was a Time and Space Legendary, I highly doubt "Anti-matter" could have been on your mind for the third.