The digital snake-thing that made its debut in Pokémon X and Y was just the tip of the five-form Zygarde
, and while we’ve seen the doglike 10 percent form and Digimon-ish complete forms in side games and the anime, we’ve yet to see them in a proper Pokémon game. Now we have.
Source: Kotaku – Watch Zygarde Achieve Its Final Form In Pokémon Sun and Moon