https://www.gamespot.com/articles/free-legendary-pokemon-coming-in-may-for-ultra-sun/1100-6458501/

Next month you'll be able to claim two more Legendary Pokemon with an in-store promotion at GameStop stores. Serebii reports that from May 4 through May 27, you can claim the dark/flying type Yveltal in Pokemon Sun and Ultra Sun, and the fairy type Xerneas in Pokemon Moon and Ultra Moon. Both were the cover characters for Pokemon X and Y, respectively.

This is part of Nintendo's year-long Legendary Pokemon celebration, which has been distributing a new set of rare monsters regularly through in-store promotions. The current promotion is through Target stores, handing out a free Entei and Raikou. That giveaway has ended for European trainers, but those in the states still have until April 29 to claim theirs.