Marilyn Manson is a famous musician/singer. Chris became a fan of his music in Saving Private Brian. As Chris spirals out of control into rebellion, Peter and Lois are concerned for their sons wellbeing. They go to a Music Awards Ceremony and bring Marilyn Manson back to their house to talk to Chris. He then convinces Chris to respect other people again and finishes the episode. Marilyn Manson was not actually in the episode. Seth McFarlane never actually asking Manson to provide his voice. Thinking it might be too mocking to Manson because in the episode Manson is purposely given a nicer definition than in real life, such as telling Chris to eat his vegetables, or the fact that he doesn't hit or even physically retaliate to Peter when he hits him.
Appearance
The Mechanical Animals cover is shown as the poster in Chris' room, and Manson's appearance is similar to what he wore in the Holy Wood era. Peter makes a reference to his appearance on the Mechanical Animals poster when he says "I'm gonna give that bastard or bitch a piece of my mind or penis."