Holy Herpetophobia, Batman

Physical deformity has played a huge role in creation of villainy. The importance we put on human beauty means the alternative, ugliness, becomes vilified. The best villains from fiction have physical deformities that mirror their evil personalities. James Bond villains all have physical issues, Dick Tracy villains are all identified by their unique malformities. Witches with warts, the one armed man, Captain Hook, Freddy Krueger, Scar from the Lion King, every movie monster ever are all good examples. Killer Croc is the most obvious deformed Batman villain.

Killer Croc was created in 1983 by Gerry Conway and Gene Colan for Detective Comics #523 where he appeared in a cameo. His first full appearance was in Batman #358 a few months later. The character was depicted as a man with reptilian-like skin. Because of his gruesome appearance, Jones pursues a career as a sideshow oddity, and then as organized criminal muscle.  In Geoff Johns Earth One story, Killer Croc becomes a Batman ally and helps found the Outsiders. Aside from this, though, Killer Croc is always a villain. 

Waylon Jones, the Killer Croc, was written as a character with a severe case of ichthyosis. This real skin disorder causes people to develop hard, scaly skin. Obviously, real sufferers of ichthyoisis don't mutate into reptile people. This Batman villain, however, is very clearly portrayed as crocodillian, often including increased size, sharp teeth, nasal deformities, prehensile tongue, and sometimes includes claws, and a tail. In order to explain this, as comic books all try and do, Killer Croc either has a super rare, perhaps, unique case of ichthyosis that continues to mutate his body in crazy ways, or he has a history with genetic manipulation at the hands of covert, illicit scientists.

Throughout his publication history, Killer Croc is portrayed as a mindless mafia enforcer, or a mob boss smart enough to organize his own racketeering schemes, or a barely-human sewer dwelling cannibal. Killer Croc has become a villain symbolic of our fear of degeneration. He has become more than just a Dick Tracy-esque crime lord with a weird skin disease, and now embodies animalistic tendencies including blind rage, brute strength, fight or flight response, and the ancient taboo of cannibalism. Essentially, Killer Croc is Gotham's resident monster, larger than life, ugly, feeding into all of our primal nightmares. His name may sound stupid, but written well, he can be terrifying.