In the anime it is indeed shown that a Slowpoke evolves into a Slowbro when a Shellder - the shell like creature, lol - bites it's tail (for in-game Slowpoke just evolves around Lv:37). There are also theories that when a Slowpoke evolves into a Slowking when bitten on it's head by a Shellder (while in-game a Slowpoke evolves into Slowking when traded with a King's Rock).
The current relationship between Shellder and Slowpoke is not clearly known by now, but at the time of the first generation of Pokemon, Shellder was the only shell-like creature. Maybe to fill some gaps in the game or Pokedex, this story in the anime has been used. :)
Also, some Pokedex entries state that when the Shellder leaves Slowbro/Slowking, they De-evolve pretty much or forget their moves (just a Pokedex statement, nothing is ever implemented about this anywhere, lol).
Therefore some questions remain like: Why would a Shellder change his form so drastically when attached to Slowbro/Slowking? Why would a Shellder bite Slowpoke's tail/head? Indeed in generation 2, it was stated by Team Rocket (in Slowpoke Well), and some other guy outside of Union Cave, selling Slowpoketail, that this is some kind of expensive delicacy.
I bet there is more to think about, but this makes these Pokemon and the concept still kinda interesting. :) When I was a kid, I thought I needed a Shellder and Slowpoke in my party in order to let my Slowpoke evolve (which wasn't the case lol).

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