From Homestar Runner Wiki
Donnie Chapman is the oldest son of Don and Harriet Chapman and brother of The Brothers Chaps. He appears in monster truck as all four members of Limozeen; these photographs were taken in 1987 when he dressed up as Vince Neil of Mötley Crüe for Halloween. These photographs made a reappearance on a Limozeen poster in the liner booklet of the Strong Bad Sings and Other Type Hits CD; this poster was once again seen in Lappynapped!. Donnie's birthday is September 28.
According to an interview with Adventure Games, Donnie influenced the making of the Peasant's Quest Movie Trailer by reading early '80s video game ads to Matt and Mike. The Brothers Chaps have taken creative inspiration from other childhood memories with Donnie. One result is, for example, Powered by The Cheat Homestar Runner's "Behan" comment in mile's commentary, as well as Strong Bad's knowledge of guitar techniques. Also, the Mysfit-steries variation of Homestar reminds Matt, Mike, and their sister Karen of the way Donnie looked in the '70s.
Finally, in the highschool DVD commentary, Matt states that "Donnie's responsible for the reason we're funny at all."