I really want to play Pokémon Go but I am a completionist. If I start playing now, what Pokémon won't I be able to catch?
edit: the list is getting a bit out of date. I do not have time right now to update it, but a lot of previous legendaries are obtainable via the 7-day research reward, or coming back into legendary raids. Also, trading is now a thing, making it a lot easier to get eveerything you missed. The only ones you currently misseed are via special quests. These quests should have been picked up (but not completed) in a certain time period. Rewards for those were:
- Meltan (but obtainable via a Mystery Box)
There were multiple limited time Pokemon, mostly legendaries. It is currently not known if they will return (all of them or a select group). These Pokemon were:
- (Shiny) Lugia
- (Shiny) Ho-oh
Currently there is another legendary active, Latios/Latias (and shiny Ho-Oh), so be sure to catch that one while it is still possible. Also Mewtwo is available through Ex-raids at this moment in time.
The legendary birds (Articuno, Moltres, Zapdos) were twice catchable. First a week per bird, then about another month where all 3 birds were available at the same time. Now Zapdos is available by finishing a research quest on 7 individual days. Moltres was already available. Articuno will be available in June.
Another 'issue' might be the special hats for the Pikachu family. At specific moments in time they were catchable/hatchable with a different hat.
One other thing that might be annoying is that there are regional Pokemon. For example, Farfetch'd is limited to Asia only. However, there has been an event where it was catchable for 48 hours worldwide. You can still catch Farfetch'd in Asia, but not so soon in your hometown (if not located in Asia) though.
The regions of Seviper and Zangoose are switched right now. Catch the one that should not be in your location!
Posting an updated answer since the old one is outdated.
Kanto Pokedex. All Pokemon are obtainable.
Jotho Pokedex. All Pokemon are obtainable.
Hoenn Pokedex. The list of unobtainable Pokemon are:
- Deoxys Speed form - Normal, Attack and Defence forms are obtainable
Sinnoh Pokedex. The list of unobtainable Pokemon are:
- Mime Jr.
- Rotom (all forms)
- Phione and Manaphy
- Shaymin (both forms)
Unknown Pokedex. Meltan and Melmetal are obtainable
No Pokemon in the Unova, Kalos or Alola Pokedex are available. However, Alolan variants of Kanto Pokemon are all obtainable.
Note that many of Pokemon that are obtainable can't currently be caught, for example Mewtwo and Spiritomb. If starting now, you'll need to trade with a player to obtain these Pokemon.