This character has been mentioned only and never actually shown in a comic book or on-screen. Despite their lack of physical appearances, information about them might still be relevant or interesting or at least worthy of documentation. Anything known about them is to be considered second-hand information.

Mad Dog was a criminal in the time of Amadeus Arkham. In Arkham's absence while perfecting his asylum, Mad Dog murdered the doctor's family. After being captured, Mad Dog was among one of the first patients of the asylum, eventually being "accidentally" killed by the maddening Amadeus Arkham.