Pachirisu was one of the results of the great Pikachu cloning scheme of '98. The cloning however went horribly right when Pachirisu became one of the few Pokemon that could absorb John Cena's DNA. It therefore became a super soldier and has been known to take down Salamence and Tyranitar.
Pro Players have been known to keep at least 6 in their house to ward off Twelvies with draco meteor. They are currently applying for a name change and mass face reconstruction to dissociate from Pikachu.