I want it! But I do not know were the cave it or how to get to it. Will someone tell me where and how to get to that cave? Please!
You go to route 18 (between Anistar & Couriway) to Terminus Cave (you can only catch Zygarde post-game).
Go south of Anistar city to route 18, follow the dirt path into tall grass, follow the path, go over the bridge, (watch out for the battle girl), and you'll find an entrance to a mine (go in).