As it turned out, Jack was still immortal when he returned to the past, spending years of his life adjusting to things. He eventually learned to speak english to the point he'd lost his natural accent. He got into science remembering the wonders he'd experienced in the future, he harnessed some remnant of the black puddle from which Aku had spawned, and dubbed 'Chemical X'. Reminded of his lost love in a feat of loneliness he attempted to create children through science.

In an accident he exposed his concoction and his pet simian to chemical X, mutating both. The simian grew in intellect, but also in ego as his peach skin turning green beneath his black fur. He developed no abilities of his own, but grew an irrational hatred to his owner… Something about that Foolish look of his.

Arm the Headcannon! but yeah, not the first to make one of these ideas up, since it could also be the reverse if Townsville really is the Ruins of the Dog Civilization. .

They predicted it all along.