Gustav is crazy skunk Madman's pink rubber pink toy who Madman apparently believes is real. Madman plays with his pig often.
In An Acorn Too Far, Madman plays with Gustav in the grass bumping him into a small rock.
In Raiders of the Lost Kazoo, Madman was doing a magic show with Gustav and sock puppets. No-Neck got tired of that. Coco decided to face the fact that maybe Abe was right and she's just overreActing. She leaped off a building and smashed a rectangular tile off of the sky. She realized she was right when Ken had his army capture her and lock her in a chair by the wrists. Nigel was selling burrito's in a cardboard box telling jokes to people. Abe realizes that Nigel just sold a burrito to the same customer he served 20 minutes ago. Coco through the cameras was cheering him on. Gustav moves to a different location which was very suspicious. On stage, Abe wrestles with Gustav and Gustav splits into two and there's computer software in there making him a robot. Madman gets a little upset. The crowd are robots and chase after them but explode when Coco takes control of the the technology systems. Ken pushes her out of the way. The GLF members are watching a movie (including Gustav) and Coco decides to sit by Gustav. Gustav's eyes turn red.
In We Won, while in the sewer Madman put Gustav up to his ear as if Gustav was telling him something and said that if they starved Gustav would rather eat Nigel than Madman. Madman pretended to have Gustav nibble on Nigel.