Pokemon GO has hundreds of Pokemon for players to find, catch, and battle. With Pokemon up to the sixth generation being implemented, there aren't too many generations left to catch up on.
Despite reaching the Kalos region, however, Pokemon GO is still missing multiple legendary and mythical Pokemon from generations that have been at least started to be implemented. While some of these may appear in future raids and events, others have more questionable statuses.
Despite being technically separate categories, many Pokemon players lump mythical and legendary Pokemon together due to their similar statuses of usually being one of a kind. With some mythical Pokemon also being related or connected in some ways to certain legendary Pokemon, the waters continue to get muddied. Both, however, will be listed.
Manaphy
Manaphy is a pure water-type Pokemon technically introduced before Pokemon's fourth generation in Pokemon Ranger, but was first made actually available in gen four. This Pokemon was given to players through a special looking egg that they had to hatch in order to get Manaphy. While the original event required to get Manaphy was a bit of a pain to deal with, future events and giveaways made getting this Pokemon much easier, but it has yet to appear in Pokemon GO.
Due to Pokemon GO having a special mechanic with Pokemon eggs, there could be an event at some point in the future surrounding Manaphy's egg. It could require more steps then any other previously seen Pokemon egg, but there is no word on this.
Phione
Phione is another pure water-type Pokemon from generation four, and it greatly resembles Manaphy due to Phione's parentage (a rarity). With Manaphy not being implemented into Pokemon GO it makes sense that Phione is also not in yet. However, this could also be due to how players are meant to get Phione in the mainline game. This involves a feature that is not in Pokemon GO, and it being unknown if the feature will ever appear in-game.
For players to normally get Phione in most Pokemon games, they would need to put both a Ditto and a Manaphy in a Pokemon Daycare. This is to convince the two to breed and make a hybrid of the two, Phione. Considering how reliant Phione's existence is on this, its unknown how this would happen in Pokemon GO at all.
Shaymin
Shaymin is yet another Pokemon from the fourth generation and is a pure grass-type Pokemon, though it can transform into its alternate Sky Form which changes its appearance and type into a dual grass and flying-type. Normally, one would think that Shaymin's dual forms would be the reason for it not yet appearing in Pokemon GO, but this likely isn't the case. This is because of Giratina, another Pokemon from the Sinnoh region, having dual forms with players just being able to catch it in either form. It can also be compared to the different forms of Deoxys since none of these Pokemon have access to the item or location that controls their form changes.
Arceus
Arceus is the final member of the creation trio and is widely considered to be the god of all Pokemon. Game lore states that it is the creator of the Sinnoh region as well as possibly the entire universe of Pokemon. Arceus's god status isn't likely what is keeping it from appearing in Pokemon GO yet, rather it could be due to its alternate forms. While certain Pokemon have usually only a couple alternate forms maximum that change their stats or typing, Arceus officially has eighteen different forms with a different form for each Pokemon type.
While Arceus's form changes in the mainline games are controlled by its held item, these are not available in Pokemon GO. With its status making it a contender for a raid boss Pokemon, it would also be unlikely that there would be eighteen variations of Arceus available in raids at once. Because of this, it is unknown when or if Arceus will be added.
Keldeo
Going into generation five, Keldeo is also yet another Pokemon not yet added into Pokemon GO. Keldeo is not alone from the Unova region, however, with other gen five Pokemon such as Larvesta, Munna, and Zorua still not having appeared. Like many of the other Pokemon already mentioned, Keldeo has two different forms. One is its normal form and the other is its resolute form. This form is activated if Keldeo knows the move Secret Sword, its signature move. It is unknown if this would be how the form change would occur in Pokemon GO or if it will simply be able to be caught in one form or another when it is finally released.
The Kalos Legendaries and Mythicals
With the sixth generation of Pokemon only now being added into Pokemon GO, there are still many Kalos Pokemon still not implemented. Since it's mostly early-game Pokemon from Pokemon X and Pokemon Y that are currently added, it makes sense that the legendary and mythical Pokemon still aren't in yet.
When it comes to it, players will be able to look forward to Xerneas, Yveltal, Zygarde, Diancie, Hoopa, and Volcanion. With Xerneas and Diancie, players in Pokemon GO will finally be able to have access to a mythical and a legendary fairy-type Pokemon, as the fairy-type was introduced in gen six.
Considering the track record when it comes to the previous generations, it might take some time for these Pokemon to actually appear for players. This delay in release seems to be more common for mythical Pokemon than legendary Pokemon, though, so Xerneas, Yvelta, and Zygarde will likely appear before Diancie, Hoopa, and Volcanion. Nonetheless, there has been no word on any of these Pokemon as of January 2021.
Of course, after Pokemon's sixth generation, none of the legendary or mythical Pokemon from either gen seven or eight have appeared in Pokemon GO. The only exception to this is Meltan and Melmetal, who actually made their first appearances in Pokemon GO instead of a mainline game.
Pokemon GO is available now on Android and iOS.