Lizard

Lost his right arm in an accident while serving in the military
Developed a serum using reptilian DNA and injected himself, causing his missing arm to regenerate
Shortly afterward, he mutated into a savage reptilian creature known as The Lizard
Spider-Man restored him to his human form, though his missing arm remained unrecovered
During moments of extreme anger or stress, his body reverted back into its reptilian state
When transformed, the reptilian side of his mind took control, turning him into a mindless beast