Pokemon GOcurrently has hundreds of Pokemon from the mainline games implemented for mobile players all around the world. While Niantic seems to be added Pokemon in order based on their generation, some Pokemon have been left behind in the shuffle.
Some of these Pokemon might not be inPokemon GOyet due to their odds methods of obtaining them in the mainline games or complexity of their signature moves. However, some of these Pokemon are from generations that have already seemed to pass by inPokemon GO.

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It is unknown when these Pokemon will be added toPokemon GOor if they ever will be added. The Pokemon still left over from Generation 5 are likely going to be implemented soon, however, before theMega Evolution update.
While some of these Pokemon exist inPokemon GO’s data as of August 2020, many of them are not available for players in-game. According to thePokemon GOwiki, the followingPokemon have not been releasedfor players at this time:

While many of these Pokemon arePokemon from Unova, making it more reasonable for their absence, some of these raise more questions than others for their absence. For example, Kecleon is from the Generation 3, but is still not inPokemon GO. This is likely due to Kecleon’s signature ability, Color Change.
Abilities aren’t inPokemon GO, but they are important to how Kecleon works. Color Change allows Kecleon to change its type to whatever move it is hit by. Due to the overall lack of move variety in Pokemon GO’s battles and no other Pokemon having abilities, this might have made the subject of Kecleon difficult for developers for Niantic.
Phione also presents Niantic with a difficult situation. Players would originally get Phione in the mainlinePokemongames by breeding a Manaphy with a Ditto in the Pokemon Daycare. SincePokemon GOdoesn’t have any breeding systems, this would make the obtaining of Phione be a mystery.
Manaphy is also in a similar situation with players usually needing to hatch Manaphy from a special egg in the mainline games. However, it wouldn’t be strange forPokemon GOto have aspecial research taskrewarding players with a Manaphy egg that requires a lot of steps.
Arceus is considered to be the God of all Pokemon, so the obtaining of Arceus would likely be through some kind of event like aPokemon GOFest. However, there likely won’t be an event surrounding Arceus until the current global pandemic has ended or at least gotten better.
While Sirfetch’d is from the seventh generation, it is still insidePokemon GO’s data. This is because of Galarian Farfetch’d already being inPokemon GOalongside otherGalarian forms of Pokemon.
Pokemon GOis available now in select regions on Android and iOS devices.