The horseshoe has aquired symbolic significance not because of its function, but because of its shape and the metal used to make it. It is shaped like the arc, one of the first sacred symbols that represents the vault of the Heavens. When it is “upside down” it is also shaped like the last letter of the Greek alphabet, the Omega.
Flip the horseshowethe other way up however, and it resembles the crescent moon, therefore invoking the portection of the Moon Goddess. The iron that the horseshoe is made from further enhances this protective quality. Iron is a protective metal, which evil entities will go out of their way to avoid. The horseshoe also looks like the yoni, further strengthening its links with the Goddess.
The horseshoe is a well-known good-luck symbol and appears on greeting cards, wedding souvenirs, and the like. People nail them up over doorways for the same reason, although there is some controversy as to which way up the horseshoe should go. One school of thought says that it should rest on its curved end to hold in the luck, which, if the horseshoe were reversed, would pour away. However, pre-Christian superstition says that the horseshoe should be positioned so that it looks like the sky, and also like you.
From the book : The Element Encyclopedia of Secret Signs and Symbols by Adele Nozedar