https://www.gamespot.com/articles/new-free-legendary-pokemon-for-ultra-sun-and-moon-/1100-6459177/

The Year of Legendary Pokemon rolls on. With June just around the corner, The Pokemon Company has shared some more details about the next free Pokemon it is giving away for Ultra Sun and Ultra Moon, and it's another Legendary from the Kalos region: Zygarde.

In contrast to previous distributions, which offered a different Legendary depending on which seventh-gen game they're redeemed in, Zygarde is the only Pokemon that players will receive in June, regardless of which version they have. What makes this particular Legendary special, however, is that it's a Shiny variant, which is exceptionally hard to get through normal play.