Dr. Curt Conners
Skilled surgeon Dr. Curt Conners lost his right arm while serving with the US Army overseas. Returning home, he developed a formula to mimic the regenerative properties of reptiles in humans, letting him grow a new arm like a lizard can grow a new tail. The formula worked too well, and ended up transforming Dr. Conners into the mammal-hating monster known as the Lizard. Spider-Man fought the Lizard, and was able to come up with a antidote to Dr. Conners' formula.
When he's not the Lizard, Dr. Conners has been a valuable ally to Spider-Man, helping him with strange mutations and other scientific matters.