It is illegal to ride a horse down the street if it does not have a taillight attached.

The Law Explained

As early automobiles started sharing the dark, poorly lit streets with traditional equestrian riders, the vast speed difference created a massive safety hazard. Rather than banning horses, lawmakers required riders to attach a reflective device or literal lantern to their animal's rear. This prevented nighttime rear-end collisions between fast cars and slow horse carriages.