Mad Dog Inukai, known simply as Mad Dog in the English dub, is a minor antagonist from the second season of Yu-Gi-Oh! GX, only appearing in "No Pain, No Game" as Zane Truesdale's underground dueling opponent.
Mad Dog was an Underground duelist, having defeated twenty duelists in a row and earned the title "Gatekeeper of Hell".
In an underground dueling bar, Mad Dog faced an arranged opponent, Zane Truesdale, who had been relegated from the Pro League following a losing streak. As the duel began, Mad Dog taunted Zane, by calling him "Pro League Dog". Both players had electrodes strapped to them, so that they would receive high voltage shocks each time they took damage. Mad Dog was aware of the cards Zane used and was able to counter them. He quickly lowered Zane's Life Points to 300 and raised his own to 15000. Zane became frustrated by his losing streak and abandoned his respectful way of dueling, in favor of more ruthless strategies. Zane Summoned "Chimeratech Overdragon" at 4800 ATK and attacked Mad Dog repeatedly, dropping his Life Points to 0, knocking him out and blasting open the cage.
- In the Japanese version, "Mad Dog" is his nickname, and translated literally, his given name means "dog owner".
- Mad Dog's voice in the 4Kids dub is a parody of Mike Tyson's vocal mannerisms.