>>38225556Completely unavailable (18)
>Bulbasaur/Ivysaur/Venusaur>Charmander/Charmeleon/Charizard>Squirtle/Wartortle/Blastoise>Omanyte/Omastar>Kabuto/Kabutops>Articuno>Zapdos>Moltres>Mewtwo>MewUnavailable pre-League (21)
>Diglett/Dugtrio>Grimer/Muk>Chansey/Blissey>Kangaskhan>Mr. Mime>Porygon/Porygon2>Aerodactyl>Snorlax>Murkrow>Misdreavus>Slugma/Magcargo>Houndour/Houndoom>Larvitar/Pupitar/TyranitarCrystal potentially makes available pre-League
>Pichu/Pikachu/Raichu (Odd Egg)>Cleffa/Clefairy/Clefable (Odd Egg)>Cubone/Marowak (Goldenrod Game Corner)>Elekid/Electabuzz (Odd Egg)>Sneasel (Ice Path)>Celebi (GS Ball event)So 212/251 Pokémon are technically obtainable pre-League if Gold, Silver, and Crystal are used with their features like trading and Mystery Gift. Of the remaining 39, 21 are truly exclusive to the post-game. And of those 21, 11 are Gen II Pokémon.
89/100 Gen II Pokémon can be obtained prior to beating the Pokémon League. The only Gen II Pokémon that can only be obtained post-game are
>Blissey (evolution of a Gen I Pokémon) >Porygon2 (evolution of a Gen I Pokémon)>Murkrow>Misdreavus (technically in Johto)>Slugma/Magcargo>Houndour/Houndoom>Larvitar/Pupitar/Tyranitar (technically in Johto)