Over the weekend, I actually came to remember a solid contender for this thread. At least in the category "Pokémon we never wanted", it has to be up there:
Chingling.
Why is that so? Well, first, some background. Namely, this guy:
In Ruby and Sapphire, Chimecho was probably the one Pokémon you'd be the least likely to catch, for three reasons:
One, it could only be found on the very summit of Mt. Pyre, where it was a 2 % encounter in the tall grass. That meant you were likely to sift through
fifty Shuppet and Duskull before finding it. Tell me, in such a far-off location as Mt. Pyre, who would want to spend enough time in that grass to find Chimecho, unless the dice rolled Chimecho's way really early? You have one reason to even visit the summit of Mt. Pyre, and that is for one story moment. Two, if you come back to return one of the Orbs, which is only in Emerald, I believe. The only other Pokémon found in the grass, Shuppet and Duskull, can also be found inside Mt. Pyre. Okay, in Ruby Shuppet is 3 % more common in the tall grass than it is inside the mountain, and vice versa for Duskull in Sapphire and Emerald, but you really have no reason to go into the grass at the summit even if you are hunting for a right-natured specimen of the version-uncommon type. The only reason you have to go into the grass at all is to find a hidden Rare Candy. But you have no reason to stick around for long enough to encounter a Chimecho. The point is, Chimecho is extremely rare.
The second reason: 65/50/70/95/80/65 = 425. Those are Chimecho's stats. It's no Luvdisc, but as far as BST goes, it's between Wormadam and Emolga (other Pokémon with that total: Marowak, Sunflora and Swoobat). Chimecho has far worse stats than other Psychic types in RSE, such as Grumpig, Gardevoir, Claydol, Xatu, Medicham (if you factor in Pure Power), or even Solrock/Lunatone. I suppose it can claim higher stats than Kadabra, but only because Kadabra skimps its Physical stats to excel on the Special side. Chimecho also lacks the moves to do anything worthwhile, with a movepool consisting of Confusion, Psychic, and a whole bunch of crappy Normal-type moves, all of which are Physical since this is Gen III. I suppose you could teach it Shock Wave if you wanted a damaging Special move that is not Psychic, but that was a one-off TM with far better mileage on other Pokémon. Your
only other option would be Hidden Power, and we all know how reliable that move is for casual play...
Point is, Chimecho is extremely weak, which makes it unattractive, and even disappointing considering the effort it requires to be found.
The third reason: Guess how many trainers in the game use Chimecho? This isn't possible to check on Bulbapedia or Serebii, but according to TVTropes it is zero. Coinciding with the fact that Chimecho has the highest index number in Gen III, which means it was the last Pokémon coded into the game, it seems like the designers all but forgot about Chimecho. It is used by nobody, not even giving a hint of its existence in the games. Of course, it was used by James in the Anime, which gave it the advertising it needed not to be the most obscure Pokémon ever until Lumineon was released.
Point is, Chimecho is extremely obscure, meaning you won't even learn about it unless you look it up.
So, why is Chimecho not the worst Pokémon ever? It's hard to find, very weak, and even hard to
discover if you didn't watch the already-then long-stale TV series. It's so hard to find, which builds expectations, yet so weak that it becomes an epic disappointment when you eventually get it.
Well, at this point, Gen III, Chimecho could possibly be the worst Pokémon in existence, but it still had potential. Chimecho's stats are slightly higher than, but overall comparable to, those of Kadabra, Metang, Duosion and Gothorita. It has lower BST than, among others: Sneasel, Misdreavus, Piloswine and Magneton. Overall, Chimecho compared well to middle-evolutions, there was no reason why its line should be capped. Gen IV was right around the corner, promising evolutions to many Pokémon of previous generations. Even Porygon2, its BST a monstrous 515, got an evolution. Apparently for the last time, Game Freak would make proper additions to evolution chains, giving a permanent upgrade to the monster previously considered the apex of its family. Chimecho had potential, Gen IV had promise, and the designers had plans for the two to converge.
So what did Chimecho get?
Gosh-darned Chingling. A pre-evolution. And that was it.
You know, Game Freak, when fans compared Chimecho to a middle-evolution, hinting that its line should longer, we didn't wish for an addition on
that end of the chain. Chimecho was already borderline useless. Chingling is even more so. Why would you even want to catch a Chingling? We all knew Chimecho is worthless, and having a pre-evolution does not make it stronger in the end. Nobody wants to catch Chimecho. Catching a Pokémon that requires actual investment to eventually
become Chimecho? Now,
that has to be among the worst Pokémon ever.