Mad Dog appears as a massive balloon dog-like puppet, with a light pink body, googly eyes and a wacky grin. She has small loops for ears, with pink stripes near the tips. For her neck and legs, the starting segments are green; However, for the legs only, the next segment colors are pink and bronze-gold, the latter being the color of some of her other segments, and her head. Her nose is an orange color, the same color as the spokes holding her wheels on. On her back is a turning key, indentifying her status as a Keymaster, and on her left and right sides is the ghastly emblem of Dark Demon.
Mad Dog is encountered in Scene 2-3, when she crushes Trouble Bruin before he could get to Headdy. In the Japanese version of the game, she boastfully proclaims she's intent on getting a bounty reward for eliminating the puppet hero, before launching a sudden attack upon him. Her main weakpoint is the green tip of her wagging tail. Like any dog, she likes to play with her ball, which she will often drop after one of her attempts to flatten Headdy. This ball will go around the screen and hurt Headdy if he touches it. Mad Dog will also drop caterpillars, balls containing smaller dogs, and bombs to try and injure the puppet hero. She has a slight chance of dropping Bino during the battle. She jumps up and crashes though the ceiling, hoping to crush Headdy.
- Her theme is a 16 bit verson of Peter Ilyich Tchaikovsky's The Nutcracker.-
- Boundy's English name most likely comes from mad dog, another word for a rabid (thus deranged) dog.