In 1936, J.E. Torbert patented a taillight for horses:

When a person is riding a horse along a road at night and an automobile approaches the horse from the rear, the signals will be illuminated by reflecting light from the headlights of the automobile and thus permit the driver of the automobile to see that there is a horse ahead of him and eliminate danger of the automobile striking and injuring the horse.

Simple enough. By that time we already had headlights for horses. What’s next?