Mad Dog McGurk is a one-shot character who only appeared in the DuckTales episode "Duckman of Aquatraz". McGurk was Scrooge McDuck's cellmate after the latter had been framed by Flintheart Glomgold and sent to Aquatraz. He disliked rich people because he himself wasn't. It is later revealed Glomgold framed him for jewel theft and was pardoned by the governor afterwards.
Mad Dog McGurk was on the surface a very tough, aloof, and uncaring prisoner, a typical bully and brute with a pessimistic streak. But underneath that, he was also somewhat childish, goofy and jockular, and reserved a good deal of loyalty and affection for the few people he truly cared for, later revealing a kind heart under a rough and uncouth exterior.
From the moment Scrooge moved into his cell, McGurk intensely disliked him because he was wealthy and well-off. He stated his dislike of rich ducks was born from envy, though being framed by Flintheart Glomgold might also have had something to do with it. Mad Dog was initially Scrooge's prison bully, who taunted him, picked on him and used him as an outlet for all his frustrations about being stuck in prison, though he did slowly gain respect for Scrooge when the latter refused to put up with his bullying, managed to best all the other inmates, including McGurk, in an arm wrestling competition and passed up a chance to get the other men in trouble with the guards. Eventually, the two became agreeable towards each other, albeit still an incongruous and irritable pair on Mad Dog's end, who had become cynical in prison.
After seeing Scrooge's loving interactions with his visiting family, Mad Dog started to feel guilty about his earlier treatment of him, realizing Scrooge was indeed a good man with a nice clan, and started to miss his own mother as well, who he hadn't seen in years. After learning about this, Scrooge decided to show him an act of kindness and arrange for his mother to visit him in prison the following day. Deeply touched by this, Mad Dog officially decided Scrooge was a friend of his and one he would show loyalty to from then on, to the point where he decided to take Scrooge with him when he tried to escape from prison sometime afterwards.
Inspired by Scrooge's courage and determination, and spurred on by his mother's visit, Mad Dog refused to give up on the idea of getting Scrooge and himself to freedom, only stopping when Scrooge himself asked him to. After divulging to Scrooge how Glomgold set him up, the pair were cornered by policemen and prepared themselves to be shot when Scrooge's family arrived - to tell Scrooge he'd been pardoned. With Scrooge's efforts, Mad Dog was later pardoned as well and free to reunite with his mother. In their last interaction in the episode, the two reassured each other they would remain friends, even outside of prison.