@Md-Martin:
I always have found it weird with Pokemon that -In their dex- are clearly merged Pokemon, but in terms of evolving it are simply level up.
Slobro and Shellder for example.
Regarding the whole Shellder, Slowpoke, and Slowbro question, my take is that when they were created, Pokemon evolution didn’t exist.
Slowbro was one of the earliest Pokémon created given that it appears as a sprite in the pitch Satoshi Tajiri and Ken Sugimori developed and submitted in 1990 back when it still called Capsule Monsters. Given that the Pokémon shown as sprites appear first in the in Pokémon index ordering (not the same as the Pokedex ordering), it’s reasonable to speculate that the index order is a roughly correlated with creation order.
Two Pokémon that also as drawings on that pitch are Staryu (#27) and Blastoise (#28) which are further down on the index list after Shellder (#23). So Shellder’s sprite was at least completed before those two. Slowpoke comes after those two at # 37, still pretty early on.
Now this is speculation on my part and others. Game Freak had internal polls to judge the designs of the various Pokémon, and for one (I think from around October 1992) that was released for outside viewing, the last Pokémon on the list that was kept was Krabby at # 78. Of the Pokémon with a lower ID #, there’s not really a hint of that evolution was going to be part of the game (besides the Slowpoke line, only the first and final forms of the two Nidos, Gastly and Gengar, and the Exeggcute line appear, and let’s be frank, the last two groups don’t look anything alike).
But when you get to # 82 with Vulpix, Pokémon are introduced with clear evolution patterns in mind. Vulpix is followed by Ninetales (#83), Pikachu (#84) by Raichu (#85), Eevee and its three forms (# 101-105), etc and also Pokémon meant to evolve from and/or into older forms start showing up. Pokémon had a long development history with the team taking hiatuses from it to work on other projects to stay afloat (Game Freak was a really small team after all and Pokémon was Satoshi Tajiri’s dream project, so of course he’d take time), and it’s likely that the idea of evolution came to be during one of these hiatuses. Pikachu's designer, Atsuko Nishida, said that when she was given instructions to develop an electric type that evolved twice, so evolution predates Pikachu, but Pikachu is stated not to have been on the October '92 poll.
In conclusion, I personally don’t think that Slowpoke was meant to evolve into Slowbro originally, but the idea that there was a relationship between Shellder, Slowpoke, and Slowbro had been established early on. That was changed when they started to create new Pokemon for evolution lines, and they decided, why not just have Slowpoke lvl up into Slowbro because it was simple to do and they had a connection.
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@MetaMario:
Hey, at least they didn't just staple more Magnemite together to make it.
This would be a fair compliant, but all evidence suggests that Magneton (who was on the October '92 poll) was created before Magnemite (who is not). Diglett and Dugtrio, on the other hand.
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@Hekti:
Wasn’t a Pokemon in the beta which was found recently with a Pokemon which looked like the Slowbro Shell?
Sort of. It’s technically the headpiece for Slowking because it has a jewel in it like you see in Indian turbans (even the pokemon’s name is Taaban, the word for turban), unlike Slowbro’s shell. But it has no real relationship in the game data to either Slowking or Shellder.