The Harley Riders are overly noisy motorcyclists. They appeared in the Season Thirteen episode, "The F Word".
Background
The Harley Rider population has increased considerably, to the point that not only the boys, but in fact the entire town has grown sick and tired of them. The riders themselves however are under the mistaken impression that when people look at them drive by, it's because they think they're cool. Eric Cartman however is the first to inform them that they are wrong, and that people in truth believe them to be complete fags.
The Riders refuse to believe him, and continued to drive around town, thinking they are cool. However, as the drive around town, they quickly realize the kids believe them to be fags. The boys and their friends meanwhile attempt to push the Riders out by writing "Fags Get Out". This later leads to the town accepting the word "Fag" to reference Harley Riders as opposed to homosexuals.
Furious, the Harley Riders attempt to stop the word "Fag" from officially referring to them, but instead accept the meaning, and the word officially becomes associated with them:
"Fag (făg) n. 1. An extremely annoying, inconsiderate person most commonly associated with Harley riders. 2. A person who owns or frequently rides a Harley."
Appearance
Although of varying appearances they all are middle aged men, wear leather vests, chaps and assorted other leather articles of clothing.
Personality
They appear to be extremely insecure, becoming upset when they are not the center of attention, and becoming violent after being called fags. They also appear to be willing to hurt others to "get their point across".