Earlier this year, Legendary Pokémon were finally introduced into Pokémon GO. They may not have been implemented in the best way, but at least they’re finally attainable in the game.
Over this past year, Lugia, Articuno, Zapdos, Moltres, Raikou, Entei and Suicune have become available in Raid Battles and Mewtwo is catchable via EX Raid Battles. Since Pokémon GO is up to the second Generation of Pokémon, the only Legendaries from the first two Generations that haven’t been introduced into the game yet are Mew, Ho-Oh and Celebi. However, dataminers have recently found assets that suggest that this may change soon.
A user by the name Chrales on TheSilphRoad subreddit recently made a post in which he or she declares that he/she has edited the “network to make my client believe my pokédex is already filled,” which allowed them to view 3D models of Ho-Oh and Celebi, which were only added into the game’s files in the most recent update. Obviously, nothing is officially confirmed, but these findings would seem to imply that Ho-Oh and Celebi might be available to catch in the game soon.
Additionally, Chrales found a 3D asset for a shiny Ho-Oh in the game’s files. There is not a 3D asset for a shiny Celebi, however, so maybe Ho-Oh is planned to be introduced into the game first. If Celebi is introduced into the game soon, however, that will make it the first traditionally event-only Pokémon to be catchable in Pokémon GO.