Pokemon GO currently 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 in Pokemon GO yet 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 in Pokemon GO.
It is unknown when these Pokemon will be added to Pokemon GO or if they ever will be added. The Pokemon still left over from Generation 5 are likely going to be implemented soon, however, before the Mega Evolution update.
While some of these Pokemon exist in Pokemon GO's data as of August 2020, many of them are not available for players in-game. According to the Pokemon GO wiki, the following Pokemon have not been released for players at this time:
- Kecleon
- Manaphy
- Phione
- Arceus
- Bisharp
- Bouffalant
- Cottonee
- Deerling
- Druddigon
- Eelektrik
- Eelektross
- Emolga
- Frillish
- Jellicent
- Kecleon
- Keldeo
- Krokorok
- Krookodile
- Larvesta
- Leavanny
- Mandibuzz
- Meloetta
- Mienfoo
- Mienshao
- Munna
- Musharna
- Pawniard
- Sandile
- Sawsbuck
- Sewaddle
- Shaymin
- Sirfetch'd
- Swadloon
- Tynamo
- Vanillish
- Vanillite
- Vanilluxe
- Volcarona
- Vullaby
- Whimsicott
- Zoroark
- Zorua
While many of these Pokemon are Pokemon 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 in Pokemon GO. This is likely due to Kecleon's signature ability, Color Change.
Abilities aren't in Pokemon 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 mainline Pokemon games by breeding a Manaphy with a Ditto in the Pokemon Daycare. Since Pokemon GO doesn'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 for Pokemon GO to have a special research task rewarding 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 a Pokemon GO Fest. 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 inside Pokemon GO's data. This is because of Galarian Farfetch'd already being in Pokemon GO alongside other Galarian forms of Pokemon.
Pokemon GO is available now in select regions on Android and iOS devices.
Source: Pokemon GO Wiki