Spider-Man 2 The Lizard

Using himself as the first test subject, Dr. Curtis " Curt" Connors injected himself with a dose of his formula and was ecstatic when his arm began to regenerate. However, the reptilian DNA his serum was based on mixed with and altered, his own DNA, and turned him into a vicious creature known as The Lizard. While Connors was eventually "cured" by Spider-Man and returned to his human appearance.