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>pokemon that will only appear in rare swarms after postgame
>item evolutions where you didn't get half the items before kanto
>general nonavailability in Johto itself
Those are the main three which are the offenders, but even the game itself wont let you use some Kanto mons.
>catch Poliwag and evolve to Poliwhirl
>have to get Kings Rock and trade to get Politoed
>Poliwrath you need a stone which you can either wait and get in Kanto at Bills old house after some hunting games or gamble on one person that calls who might give you a Water Stone or ramble some bullshit
Elemental stone based evolutions that they put in are severely gimped due to its bad availability, so even if you found a Growlithe or something, tough shit. If you found a Water Stone in Seafoam or the Fire stone in Burned/Bell tower, it wouldn't have been so bad.