Zygarde

Zygarde is a Ground- and Dragon-type Legendary Pokemon known as the Order Pokemon. It doesn't evolve, but it has three forms. Its 10% form is in the shape of a dog, its 50% form is in the shape of a serpent and its 100% form or Complete form is based on Hel from Greek mythology.
About.
Zygarde looks after the ecosystem. If it falls into chaos, it will appear. Some say that it monitoring those who mess up the ecosystem and that it will show up and reveal its secret power. It can change forms by changing the number of cells it has. 
Its 10% form, introduced in Pokemon Sun and Moon, uses up 10 percent of its cells. It can run up to 60 mph and finish foes off with its sharp fangs and skilled close combat. However, this form cannot last long.
Its 50% form was introduced in Pokemon X and Y. 50% of its cell have been gathered in this state. It wipes out foes without even a shred of mercy. People in Kalos thought that there was a greater power that lies hidden within it.
Its Complete form has the power to overwhelm both Xerneas and Yveltal and can destroy all who go against it. Its appears when all of its cells come together to form one being. From the orifice on its chest, it radiates high-powered energy that eliminates everything.