"White Horses of the Plague"
Four Riders of the Apocalypse
This reference is not clear. It seems to refer to the Plagues that “coursed Europe” during the Middle Ages, though the plagues are not normally associated with horses. It may conflate the plagues of the past with the Four Horsemen of the Apocalypse, the forces of destruction described in the Bible’s Book of Revelation; but there, only one horse is white, the others are black, red, and pale green. The woodcut at right is by Albrecht Dürer (1471-1528).
Though the other three clearly are destructive (usually taken to represent famine, war, and death), there has been considerable disagreement as to whether the white horse represents evil or righteousness. Whatever the case, they are intended to be evil in Bradbury's story.