After cutting a groove (see splitting chisel) and forming the nail holes in the horseshoe (see this stamp hammer), the farrier uses a horseshoe pritchel to punch through or enlarge the holes.
The horseshoe pritchel is a long (approx. 25-40 cm) iron chisel that ends in a small rectangular (approx. 3 mm by 1.5 mm) point.
See also the clinch cutter. [MOT]