Get ready for a surprise: The Legendary Pokémon Ho-Oh will return to Raid Battles in Pokémon GO on May 19! A powerful Fire- and Flying-type Pokémon originally discovered in the Johto region, the Rainbow Pokémon Ho-Oh will appear in Gyms across the globe alongside Latias and Latios, which will remain in their designated regions. Plus, if you’re fortunate, you might be able to encounter a Shiny Ho-Oh after you win the battle! You can challenge Ho-Oh until June 7 and seize the opportunity to add this Legendary Pokémon to your Pokédex. So, gather your strongest party to take on Ho-Oh before it returns to the skies!
—The Pokémon GO team
Source: Official Pokémon GO blog