The ornamental design for a horsecover, as shown and described.
FIG. 1 is a top side view of a horse cover of the present invention;
FIG. 2 is a bottom side view of the horse cover;
FIG. 3 is a front side view of the horse cover;
FIG. 4 is a back side view of the horse cover;
FIG. 5 is a right side view of the horse cover; and,
FIG. 6 is a left side view of the horse cover.
The broken line portions of the horse cover are included for the purpose of illustrating portions of the article that form no part of the claimed design.