I was trying to think of what Zygarde is supposed to be. It's such a strange looking Pokemon. I was looking at it, the "fans" coming off its body, how flat its body actually seems. Then it hit me...

Zygarde is supposed to be a planarian flatworm with split heads. Planarians are one of the toughest organisms in nature. You can cut them in half in pretty much any direction and both halves will grow into a new worm. A neat biology lab trick is to cut the head in half about halfway down the body. Both haves grow into a new head. You cut these new heads in half, etc, etc, and eventually you get a planarian hydra.

The fans on the back of Zygarde's back look exactly like the head of a planarian. Its body is flay, and its "head" in the centre of its body is just a mouth.