This month's CoroCoro has revealed multiple new forms for Zygarde alongside announcements for a new anime series called "Pokémon XY&Z" and the next Mega Evolution special.
Zygarde takes different forms depending on how much power it has. The small green blob we've been seeing glimpses of is the Zygarde Core, and the similar flat variant is a Zygarde Cell. Each Core or Cores makes up the "brain" of a Zygarde, while the Cells make up the rest.
As Zygarde rises to 10% power, it takes on a dog-like form, which has sharp teeth and can move at incredible speeds.
At 50% power, it reaches the form we've come to know from X&Y, the flat snake-like Pokémon.
Finally, at full power, it reaches its Perfect Form. Zygarde's Perfect Form is classified as a System Pokémon, is 4.5m/14¾ft tall (somewhat shorter than its previous form), and retains its Dragon/Ground type. It reportedly exceeds Yveltal and Xerneas in power.
Click for larger The odd-looking Greninja is supposedly a purely cosmetic change for the anime to make it look more like Ash.