A dick rider is a person who is usually and overzealous mindless fan of anyone(be it a rapper, singer, athlete, music genre, band, or any given thing). A dick rider see's no flaw in whatever they are dick riding at the time, and if you happen to offer an opinion or simply not like whatever a dick rider likes, the dick rider will consider your a "hater" for offering your opinion.
Most of the times dick riders are female fans of a band, rapper, or singer who constantly praises the graces of a music artist they never met, or are going to touch in that star's fame . Male dick riders are usually men who want to be or picks up the "swag" and attitude of their favorite artist, and they'll swear that's how they are(this is usually a phenomenon in the hip-hop culture)
David - "Jamal is such a Dick Rider