A horseshoe was a long, thin, flat piece of metal, shaped like the letter 'U', which was nailed to a horse's hooves as a protective measure.
History and specifics[]
In some Human cultures, horseshoes were traditionally hung on the walls of homes as a token of good luck. A horseshoe was hung on the wall of the Happy Bottom Riding Club on the USS Enterprise-E. (TNG novel: Q & A)