I have looked at the encounter tables to find which Pokémon are
unobtainable.
Excluding fossils, special encounters (e.g., Drifloon, Rotom), and gift Pokémon (e.g., Riolu), the following Pokémon
do not appear in any encounter table, so the only way to obtain them would be via in-game gift (AFAIK there isn't a list for this yet), event (there was a Tangrowth event in D/P), and/or special mechanic (Feebas was only catchable in 4 fishing spots in Mt. Coronet):
- Starters
- Tangela, Tangrowth
- Feebas, Milotic
- Tropius
Excepting Feebas, these Pokémon were not obtainable in the original D/P games. It makes sense to keep starters until Pokémon HOME compatibility allows transfers, but I am really puzzled by the exclusion of Tropius and (most notably) Tangela (who got an evolution in Gen IV)/Tangrowth (who we've seen in the trailers). Perhaps an oversight that will be fixed on the day-1 patch?
I am also terrified at the prospect of Feebas being locked under the crazy fishing spot mechanic. I am guessing this feature is still in, but coded differently so it is not reflected on the encounter table. Oh my, back in the day I spent hours trying to catch Feebas and I never succeeded. The only one I ever got was hacked using an Action Replay. Ugh, this is the worst mechanic in the games.