Shear off nuts
Shear off nuts, also sometimes known as shear nuts or breakaway nuts, are conical nuts that have a regular hex nut section at the end of each piece. This piece is designed to break off when the nut is torqued, shearing off the hexagonal portion, and leaving a metal cone that is hard to remove with regular tools.
Since the fasteners are often a security weak point on a fence, this helps to make your fence more secure.