Countless crooks collided with the caped crusaders of Gotham City. These two were men without superpowers only their odd appearance and demeanor, as well as sheer brutality.
Killer Croc was originally Waylon Jones, an orphan who suffered from a hideous skin affliction making him look reptilian in nature. Adopting this has is modus operandi, Jones as Croc sought to take over Gotham and knew he'd have to first deal with its protect, the Batman of Earth-One. Using his superior strength and ingenuity of unleashing several supervillains on the City all at once which its superhero protectors would have to deal with, Croc was hoping to wear down Batman. He did, for awhile, and in the interim among Jones' victims were the circus acrobat parents of Jason Todd, who would become the second Robin. Eventually, with help from Batgirl, Robin and others, Batman was able to defeat Killer Croc. Around this time, Batgirl was facing her own threats in Doctor Voodoo and his mentor, which she was able to deal with after a few battles.
Croc's counterpart on Earth-Two was a mental patient known only as Milo, who had fangs for teeth and similar strength. Milo alongside Doctor Tarr and Professor Fether... the dopplegangers of Voodoo and his partner... ran a scheme whereby Gotham's crooks would be funneled out of the City with their ill-gotten gains through a mortuary front. However, the Huntress ultimately tracked the terrible trio to Arkham Asylum, and defeated them when she unleashed the Asylum's residents upon them.
Neither Croc now Milo returned after they were humbled by the Dynamic Detectives of twin Gothams.