On April 1, 1961, Milan's La Notte
newspaper reported that city authorities had passed a new law making it mandatory for horses to be outfitted with signaling and brake lights
while being ridden through the streets or neighboring countryside. Back then, quite a few people in the area still rode horses, so the law was going to have quite a broad impact. And, so the story goes, many people subsequently brought their horses into car mechanics to have them outfitted with the necessary lights.
This is considered to be one of Italy's classic April Fool's Day hoaxes. And, as is so often the case, it's only a matter of time before reality eventually catches up with satire.
No city has passed a law requiring horse tail lights. However, over on KickStarter Sami Gros is trying to raise money for what she describes as the "world's first LED lighting system for horses,"
aka Horse Tail Lights. Unfortunately, it looks like the lights are only designed to increase visibility. They can't be used to indicate turning or braking. But perhaps future versions of them will!