With legendary Pokémon, Xerneas already making its first appearance in five-star raids, Yveltal, the legendary destructive Pokémon from the Pokémon Y will make its debut very soon. According to the recent May newsletter from Pokémon GO, Yveltal appears in Pokémon GO on May 18th. From Tuesday, May 18, 2021 at 10am to Tuesday, June 1, 2021 at 10am, Yveltal the destructive, legendary Pokémon, will be available for players to battle in five-star raids and chance to catch it. Additional details of Yveltal’s event to come soon.
Source: Pokémon GO’s May Newsletter