The Demon Dog's appearance was that of a medium-sized dog with shaggy black fur and white eyes. It is black in Gunnm: Martian Memory.
Despite being composed of plasma, the Demon Dog appeared to be under Zapan's control, displaying a degree of sentience. It was practically invulnerable to attack and very fast.
After Zapan appeared in Bar New Kansas, he threatened to kill Master when he was stopped by Glory. Murdock then appeared with the rest of his dogs, commanding Fury to protect Shumira and Koyomi, which allowed them and Master to escape. Zapan then formed the Demon Dog to counter Murdock's dogs, and Murdock believed it to be the dog of legend. Although attacked simultaneously by Valor, Glory, and Hubris, the Demon Dog was too fast for them, tearing through all three before disappearing.
The Demon Dog appears in Gunnm: Martian Memory as an enemy character after Zapan obtains the Berserker Body. They are spawned from thunderbolts and will self-destruct after their health is taken down to a certain level. It is later revealed in the game that they can be spawned as a result of Berserker cell technology. After Potemkin is infected and transforms, he summons a pair of Demon Dogs to fight Gally.
The Demon Dog was named in reference to a legendary supernatural dog of British folklore that is known as "Black Shuck" in Norfolk, "Gurt Dog" in Somerset, and "Pooka" in Ireland. In all cases, the dogs prey on humans and disappear in a thunderbolt which smells of sulphur.
- Black dog (ghost) on Wikipedia