Question

I understand that the slowpoke family is the only Pokemon that can "unevolve," but is caused by the removal of the Shellder from the head/tail, which is impossible in the game.

~I'm wondering if the Shellder technically evolves with the Slowpoke
and in the anime, what happens to the Shellder when it is removed from tail/head.

Answer 1

During the time, shellder was the only shell based Pokemon that wasnt evolved. When it clamps onto slowpokes tail, shellder needed to evolve, or slowbro would look strange. But since clamperl(shellders evolved form) would make slowbro look wierd as well, shellder instead evolved into a spiral shell. It was still a shellder.
EDIT: shellder evolves into cloyster.
Source: watching the indigo league anime. It has a reason as well.

Comments

Shellder evolves into Clamperl now, seems legit.

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lol didn't see your answer while I was writing.

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Well you could have started writing one second before I posted my answer.

Answer 2

Cos They wanted it to be like that!

Well, it's just one of GAME FREAKS/Pokemon company's mysteries. Obviously in the Game Shelder has nothing to do with Slowpokes evolution. They just made shellder bite onto Slowpoke to evolve to make it sound interesting. But Imagine the tiny Shelder biting onto the sizeable Slowbro? It would look really out of proportion. so they changed it's apperance to make it seem cooler and less..weird.

Hope I helped! PX;}