mouth.

Hang a horseshoe on one nail only, its horns will face downward. To hang a horseshoe horns up requires the use of two or more nails, or perhaps a string pulled through two holes in the shoe and hung on one nail, as if you were hanging a painting. This requires more work and more cost (during times when nails were a bit more costly than today). Hence, good luck comes to those who work harder (and spend more) for it.

My father once told me he found a horseshoe on the street as a young lad in Vienna, and brought it home. His mother was so pleased that she encased the shoe in velvet and they hung it on the wall (horns up of course). So the superstition was all across Europe.

But if you prefer a story about Saints and Devils, read this one:

https://www.wideopencountry.com/heres-people-hang-horseshoes-doors/