https://www.gamespot.com/articles/free-legendary-pokemon-available-now-for-ultra-sun/1100-6459355/

June is officially underway, which means another free Legendary Pokemon is now available for Ultra Sun and Ultra Moon. The Pokemon Company is continuing its Year of Legendary Pokemon giveaway with another limited-time freebie for all seventh-gen Pokemon games, and this time it's another Legendary from the Kalos region: Zygarde.

Unlike all of the past distributions this year, which offered a different Pokemon depending on which version you're playing, all players will receive the same Legendary this time around. What makes this Zygarde special, however, is that it's a Shiny, which is very difficult to obtain through normal play.