Horse-shoes

were at one time nailed up over doors as a protection against witches. Aubrey says, “Most houses at the West-end of London have a horse-shoe on the threshold.” In Monmouth Street there were seventeen in 1813, and seven so late as 1855.

Straws laid across my path retard;
The horse-shoes nailed, each threshold's guard.

Gay: Fable xxiii. part 1.

It is lucky to pick up a horse-shoe.

This is from the notion that a horse-shoe was a protection against witches. For the same reason our superstitious forefathers loved to nail a horse-shoe on their house-door. Lord Nelson had one nailed to the mast of the ship Victory.

There is a legend that the devil one day asked St. Dunstan, who was noted for his skill in shoeing horses, to shoe his “single hoof.” Dunstan, knowing who his customer was, tied him tightly to the wall and proceeded with his job, but purposely put the devil to so much pain that he roared for mercy. Dunstan at last consented to release his captive on condition that he would never enter a place where he saw a horse-shoe displayed.

Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894

Horse-vetch

Horse-radish