Zygarde 50% Forme

Zygarde 50% Forme is a large serpentine Pokémon. Most of its back, head, and neck are black, while the underbelly and the tip of its tail are green. Its eyes are made of four green hexagons, similar to an insect with compound eyes, and there is a thin, white line down the center of its face. Across the back of its neck is a large frill made of five long, flat protrusions that each have two small, green hexagons on them. There are three short tendrils on either side of its chest; they are black on the right and green on the left. Its tail also ends in five tendrils. It has several small, green hexagons on its body that occasionally glow white.

Zygarde's 50% Forme is the result of half of its Cells and up to 3 Cores reuniting, and is considered its main form. Its power is capable of overwhelming anyone who stands in its way, but it is only at half of its full potential, and an equal amount of power waits to be assembled out in the world. There are rumors that an even greater power lies hidden within it, as it is capable of changing to a more powerful form when battling those who threaten the ecosystem. The 50% Forme is known to be both ruthless and merciless to its foes. Its frill can be shown vibrating as it attacks. As seen in Down to the Fiery Finish!, Zygarde 50% Forme can summon and control massive roots for attacks and covering areas.

(Description copied from Bulbapedia)