Horseshoe nails
The special flat nails the farrier uses to fix the shoe through the insensitive hoof wall.
What it is for
The nails hold the shoe in place. They are driven at an angle outward so the point comes out through the wall, safely away from the sensitive tissue.
More detail
A misplaced nail that comes too close to the sensitive tissue is called a nail bind or nail prick and can make the horse lame. This is why the work belongs to the trained farrier. The protruding points are cut off and bent over into clenches that lock the shoe in place.
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Sources: British Horse Society · Merck Veterinary Manual.