People often talk about Pokémon that are forgotten or rarely used because they suck, but what about Pokémon that are simply rarely ever obtainable because they're barely even in the game? Yeah sometimes they're catchable POST-game, but by that point you're probably just gonna stick to your champion team and maybe some legendaries. They're just there for Pokédex completion.
I think a good example of this is Yanma, who despite being around for a long time, barely appears.
Gen 2: one route and ultra rare, so most will never find him
Gen 3: unobtainable
Gen 4: only in platinum (like most new evolutions for some reason), post game in DP
Gen 5: post game only
Gen 6: actually common for once
Gen 7: unobtainable except post game in ultra
Gen 8: unobtainable

Any other examples of Pokémon who rarely appear within the game's story?