The pokedex describes slowpoke as being extremely slow and barely moving, often staring off into space. Aren't psychic types supposed to be smart? So why isn't slowpoke? The theory is that slowpoke is omniscient, and that means that slowpoke has so much knowledge that it is unable to process it, and stares off into oblivion trying to comprehend all of this knowledge. When slowpoke is bitten on the tail by a sheller, it is said that the small bite of poison is able to shock slowpoke back into reality and it evolves into a useful, smart psychic type. (Although I still do wonder why slowbro got a Mega and slowking didn't).
Pokemon Myths and Theories
By theturtwiggal
An explination. More