This is one of the most common of all good luck charms. An old, used one is what should be chosen not a new one. It should be put up on the wall with nails going through the existing holes and pointing upward, which is to diffuse the energy toward the sky rather than burying it in the soil. It is also said that you find a horse shoe on your path, you should spit on it and make a wish, then throw it away, over your left shoulder and leave with no turning back. Your wish would then rapidly be answered.
The reputation of the horse shoe comes from the Roman civilisation because of its lunar shape, seen as a very beneficial shape, and its material, iron, which is seen as having the ability to repel negative energies of bad luck. This symbol has traversed the centuries and is still active now, in modern times.