Mad Dog is Bunny's abusive boyfriend and the main antagonist of the episode "The Mask".
Personality
The archetypical gangster, Mad Dog is the leader of a shifty gang of dogs living on the Wrong Side of the Tracks (which is an actual railroad track). He is narcissistic and deluded enough to believe his tattered apartment is evident of his first-rate lifestyle, and is willing to harm or kill anyone who challenges his way of life.
With this, Mad Dog is abusive towards Bunny and threatens to bury her and Kitty if the latter ever returns. However, it appears that Mad Dog has been charming enough in the past to keep coaxing Bunny to stay with him; at other times, he simply threatens Bunny with her life if she ever tries to leave him. Mad Dog also appears to be an intensely jealous character who resents Bunny's relationship with Kitty, although this is in part due to the pair's romantic relationship.
Mad Dog keeps an oversized flower pot in his living room, which was likely used to torture his victims. As a testament to his brutal nature, Mad Dog shows no qualms about burying his girlfriend in the pot.
Appearance
Mad Dog is a Doberman Pinscher breed; as such, he is muscular with a dark brown and red colored fur coat with dark red eyes. He wears a dark yellow muscle tank and a dark brown wolf collar with a circular pet I.D. tag engraved with the initials "M.D.".
History
At some point, Mad Dog and Bunny started a relationship that became abusive overtime. When Kitty learned of this and started convincing Bunny to leave Mad Dog, he threatened to kill Kitty if she ever appears before them again. This causes Bunny even greater despair, who becomes increasingly dispirited as a result of ardently missing her friend and resentment towards Mad Dog. Mad Dog's jealousy results in him threatening to kill Bunny and Kitty, as he realizes that Bunny's feelings for Kitty are incomparable to her feelings towards him. When Bunny begins to cry, Mad Dog reassures his girlfriend that he still loves her before he realizes he was tricked, as Bunny looped the carabinier on his collar to a lamp behind him. However, his gang catches Bunny when she attempts to escape through the door and buries her neck-deep in a giant flowerpot.
Later, Mad Dog rushes back upstairs when he hears the window shattering to see Bunny escaping with Courage. Once they begin running towards the train, Mad Dog chases them down in his car and almost runs Bunny down when Courage dives through the window and spins the car onto the railroad tracks. After the approaching Amtrak passenger train destroyed the car, Mad Dog is seen uncontrollably laughing in pain at his defeat on the train's now damaged locomotive.
Quotes

"Come on, Bunny. Don't I make you happy no more? Or maybe you're still thinking about Kitty."
"I take you from a two-bit joint and make you a class act, and you want to make me second rate? If I even SMELL Kitty, I'll bury the two of ya."
"Aw, Bunny, you know you're my girl. Let's go back to the way we was - you know, happy-like."
"Keep ahold of her!" (Mad Dog orders his goons to bury Bunny in the flowerpot)
"BUNNY!" (last line before his defeat)
Trivia
- Mad Dog's car appears to be based on a Lincoln Continental Mark VI.
- It is unclear exactly why Bunny fell for Mad Dog
- Mad Dog is based on the Doberman Pinscher dog breed, which are characterized for being tough and are often used as guard dogs.
- The term "mad dog" commonly refers to a dog that misbehaves, especially one that has rabies. While Mad Dog shows no symptoms of having rabies, he is cruel, aggressive, and short-tempered.
- Mad Dog has a reputation as one of the scariest villains in the series not only because of his character's appearance, but his realistic characterization as a domestic abuser.
Appearances