BEAST

In 1972, Hank McCoy quit the X-Men to do genetics research at the Brand Corporation — and promptly isolated the hormonal extract that causes mutation itself. When his superior, Carl Maddicks, schemed to steal top-secret research, Hank could not stop the theft as himself without exposing the work. So he drank his own extract, mutating further on purpose so nobody would recognize him while he intervened.

The disguise had an expiry time of roughly one hour. The furred, fanged form was supposed to be temporary, but the fight over the stolen research ran long, the window to reverse the change closed, and the transformation locked in permanently — grey fur at first, with pointed ears, claws and a healing factor he had never asked for.

Rather than tell anyone, Hank went home and studied stage make-up. He built himself a latex mask of his own former face, matching gloves for his hands, and a harness of straps to force his newly hunched posture upright — then walked back into Brand the next morning dressed as the man he used to be, and kept the secret even from the X-Men.