Lizard

Lost his right arm in an accident while in the military
Created a serum using reptile DNA & injected himself, his arm re-grew
Shortly afterwards, he mutated into a vicious reptilian creature known as The Lizard
Spider-Man was able to turn him back into his human form, minus the arm
In times of anger, stress or sadness, his body would revert back into his reptilian form;
Reptilian part of his brain takes over, becomes an unthinking beast