The ornamental design for a pet harness, as shown and described.
FIG. 1 is a front elevation view of a pet harness, shown at a reduced scale and in a first configuration with the straps unbuckled;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a top, front, left perspective view of the pet harness of FIG. 1, shown at an enlarged scale and in a second configuration with the straps buckled; and,
FIG. 4 is a bottom, rear, right perspective view thereof.