With its first Special Research quest, Pokemon GO added its first mythical Pokemon to the game in the form of Mew. However, Mew isn't the only mythical Pokemon in existence, and assuming Pokemon GO looks to add every Pokemon to the game eventually, then there are quite a few more mythical Pokemon on their way.
Mythical Pokemon, for those who may be unaware, are extremely rare creatures that can usually only be obtained through special events. So far the only mythical Pokemon in Pokemon GO, Mew, was distributed through a Special Research quest, so it seems probable future mythical Pokemon will be added this way as well.
With that in mind, here are the mythical Pokemon that are still on their way to Pokemon GO.
Generation 2
Like generation 1, the second generation of Pokemon only has one mythical Pokemon to take note of, and that's Celebi. Celebi is a psychic/grass-type Pokemon that, while not quite as recognizable as Mew, has made numerous appearances in the Pokemon anime, video games, and even in other franchises, like Super Smash Bros. Datamining efforts have already found evidence of shiny Celebi in Pokemon GO, so fans should be able to count on it coming to the game sooner rather than later.
Generation 3
The third generation of Pokemon introduces two new mythical Pokemon to the franchise: Jirachi and Deoxys. Jirachi is a steel/psychic-type, whereas Deoxys is a pure psychic-type. Deoxys is particularly interesting, as it introduced the concept of different physical "formes" for the same Pokemon, with its appearance changing based on which game it's being used in. It's unclear how that particular trait will work for Deoxys in Pokemon GO, but we do know that the Pokemon is already in the works for the game, thanks to dataminers.
Generation 4
Since there isn't a single fourth generation Pokemon in Pokemon GO as of the time of this writing, fans shouldn't expect to see its mythical Pokemon added to the game any time soon. That being said, generation 4 will add more mythical Pokemon to Pokemon GO than any generation before it, including the water-type Pokemon Manaphy and Phione (the latter of which may not even technically be a mythical Pokemon, as there is contradicting information about its status), and the dark-type Pokemon Darkrai.
Generation 4 will also look to add Shaymin to Pokemon GO, which is another Pokemon with different formes. Land Forme Shaymin is a grass-type Pokemon, whereas Sky Forme Shaymin is a dual grass/flying-type. Again, it's unclear how this will work in Pokemon GO, and we'll just have to wait for the Pokemon to be added to the game to find out for sure.
Finally, gen 4 will add the mythical Pokemon Arceus to Pokemon GO. Arceus is arguably the "God" of the Pokemon universe, confirmed to having created at least some of the regions seen in the franchise, and having been born before the creation of the universe. Due to its significant role in the Pokemon universe, the addition of Arceus to Pokemon GO is likely going to be treated as a particularly special event.
Generation 5
Generation 5 will introduce four more mythical Pokemon to Pokemon GO, including the psychic/fire-type Pokemon Victini. It will also add Keldeo, a water/fighting-type Pokemon, and Meloetta, which can be both normal/psychic- and normal/fighting-type depending on its forme. Finally, generation 5 is also expected to add the bug/steel-type Pokemon Genesect to the game as well.
Generation 6
Generation 6 is likely years away, but it will also bring new mythical Pokemon to Pokemon GO. Among the generation 6 mythical Pokemon are Diancie, the rock/fairy-type Pokemon notable for being the only mythical Pokemon with the ability to Mega Evolve. Generation 6 will also add Hoopa that, like some of the mythical Pokemon before it, has two formes. Hoopa Confined is a psychic/ghost-type Pokemon, whereas Hoopa Unbound is a psychic/dark-type Pokemon. Volcanion is also coming to Pokemon GO as part of gen 6, and is a especially unique mythical Pokemon due to it being a dual fire/water-type.
Generation 7
Three mythical Pokemon have been added as part of the seventh generation of Pokemon so far, but it's possible more could debut between now and generation 8. In the meantime, the Pokemon that have been added include Magearna, a steel/fairy-type, as well as Marshadow, a fighting/ghost-type. There's also the recently revealed Zeraora, who is an electric-type mythical Pokemon that was just revealed for the franchise a couple of months ago.
Over time, all of the mythical Pokemon listed here will likely make their way to Pokemon GO. However, when one considers the fact that it took Niantic nearly two years to add Mew to the game, it seems like players will have to wait years before all the mythical Pokemon are available in Pokemon GO.
Pokemon GO is out now for iOS and Android mobile devices.