The ornamental design for a horseshoe, as shown and described.
FIG. 1 is a top and side perspective view of a horseshoe showing my new design;
FIG. 2 is another top and side perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a side elevational view thereof from the left of FIG. 3, the side elevational view from the right of FIG. 3 being the mirror image thereof; and,