"They both read to the child extensively from the Book of Revelation."

The horses represent the Four Horsemen of the Apocalypse from the Biblical book of Revelation.
Creative Commons AttributionThe horses represent the Four Horsemen of the Apocalypse from the Biblical book of Revelation. - Credit: Rich Man / Rich
 Revelation is the last book of the Bible, written by John of Patmos (who may have been the apostle John).

It is also called Apocalypse and concerns God’s last intervention in human affairs. The book is rich in symbolic imagery, much of which can be found in Good Omens.