Definition
Conquest or Pestilence, one of the Four Horsemen of the Apocalypse.
Origins
A reference to the Four Horsemen of the Apocalypse, as the rider of a white horse is Conquest/Pestilence.
How People Actually Use It
A derogatory word for a homosexual, depicting sloppy ejaculation during male sodomy. see homosexual and gay.
"Look, Richard Simmons is on TV. He's such a White Rider."
Source: Urban Dictionary