Oh, here's a fun one:
Chimecho. Notable for being possibly the least notable Pokémon in Gen III. No trainers used it in Ruby and Sapphire, and if not for the fact that James prominently used it in the Anime, few people would even notice its existence. It's found only on the summit of Mt. Pyre, where it is stupidly rare in a patch of grass that otherwise appears to only contain the same two Pokémon as the rest of the mountain.
But assuming you look it up, hunt it down, and catch it, then what? Congratulations, you've caught one of the least useful Psychic types in the game. You're going to have to rely on Confusion for offense from the relevant attacking stat until level 46, unless you want to take your chances with Hidden Power or spend the Shock Wave TM (those are your only two options for Special attacking moves unless you breed Dream Eater onto it).
Stat-wise Chimecho faces some competition from Grumpig, which has slightly lower SpA, Defense and Attack (5 less in each of those stats, to be precise), but vastly higher stats everywhere else. Xatu is also available by the time you get to Chimecho, and it has 10 lower SpD but otherwise matches or outmatches every one of Chimecho's stats. This is also one of those games where Kadabra appears with its "Select Psybeam to win the battle" stats, even without being fully evolved. Claydol is also another Psychic attacker with more bulk, more speed, and a better moveset, or there's Medicham if you want something that's only nominally Psychic but overall a better Pokémon. Granted, Chimecho is technically available before Medicham and Claydol, by a few levels, and it's better than Meditite and Baltoy, but it's generally not a worthwhile upgrade. It doesn't offer any amazing utility in the upcoming few fights before Baltoy and Meditite evolve. And anyway ... Kadabra.
So Chimecho represents a significant investment in time of trying to find it (if you know about it at all), doesn't offer a comparable advantage in battle over more easily available alternatives, and doesn't really offer any other advantages either. Naturally, it needed an evolutionary relative. Game Freak appeared to realize this too, so when Gen IV rolled around ...
... well, damn.

Chimecho. Notable for being possibly the least notable Pokémon in Gen III. No trainers used it in Ruby and Sapphire, and if not for the fact that James prominently used it in the Anime, few people would even notice its existence. It's found only on the summit of Mt. Pyre, where it is stupidly rare in a patch of grass that otherwise appears to only contain the same two Pokémon as the rest of the mountain.
But assuming you look it up, hunt it down, and catch it, then what? Congratulations, you've caught one of the least useful Psychic types in the game. You're going to have to rely on Confusion for offense from the relevant attacking stat until level 46, unless you want to take your chances with Hidden Power or spend the Shock Wave TM (those are your only two options for Special attacking moves unless you breed Dream Eater onto it).
Stat-wise Chimecho faces some competition from Grumpig, which has slightly lower SpA, Defense and Attack (5 less in each of those stats, to be precise), but vastly higher stats everywhere else. Xatu is also available by the time you get to Chimecho, and it has 10 lower SpD but otherwise matches or outmatches every one of Chimecho's stats. This is also one of those games where Kadabra appears with its "Select Psybeam to win the battle" stats, even without being fully evolved. Claydol is also another Psychic attacker with more bulk, more speed, and a better moveset, or there's Medicham if you want something that's only nominally Psychic but overall a better Pokémon. Granted, Chimecho is technically available before Medicham and Claydol, by a few levels, and it's better than Meditite and Baltoy, but it's generally not a worthwhile upgrade. It doesn't offer any amazing utility in the upcoming few fights before Baltoy and Meditite evolve. And anyway ... Kadabra.
So Chimecho represents a significant investment in time of trying to find it (if you know about it at all), doesn't offer a comparable advantage in battle over more easily available alternatives, and doesn't really offer any other advantages either. Naturally, it needed an evolutionary relative. Game Freak appeared to realize this too, so when Gen IV rolled around ...

... well, damn.