The Horsemen were either the commanders of the armies of a race from another universe similar to the Time Lords known as Time Tyrants. It is unknown if they actually ruled the entire race. The Horsemen based themselves on the Four Horsemen of the Apocalypse and planned to regain their race's ability to travel through time after they lost their equivilant to the Eye of Harmony in their own Time War. They planned to use a Time Lord to power their own stellar manipulator to destroy Earth and conserve the energy produced to create another power source to allow them to travel in time again, rather than relying on vortex manipulators and dimensional transporters.