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As a Slowpoke it fishes for food with its tail only to have an unwanted Shellder bite it and evolve. Over time the Shellder eats the tail (Which regenerates back) and grows bigger. The Slowpoke's brain is too slow to process the pain and ignores the Shellder.
Mega Slowbro is the end result of the Shellder growing so big it eats Slowpoke whole body. Its face is sad because the information that it has become food has finally entered its brain after many long years of the Shellder being on its tail. It could have prevented its death but it was too slow to do so.
Slowpoke's destiny is to die a slow, gruesome, and agonizing death. Eaten alive by its "bro"

We will cause it to go mega.
We will cause it this pain.
We are all horrible.

or it wanted to be a ******** beyblade

Beyblades are cool, yo. Who cares if your tail-parasite devours you in order to be one?

lol good theory

Wait...Slowbro has a mega evo now?!

*checks*

Oh good lord. Who decided this?! It looks as though its silently suffering in an unimaginable amount of excruciating pain, anguish and woe. gonk

I'll save my Slowbro from this horrid fate and just not use its stone. xD