The ornamental design for an X-strap dog harness, as shown and described.
FIG. 1 is a front view of an X-strap dog harness showing my new design; and,
FIG. 2 is a rear view thereof.
The dash-dot broken lines represent the bounds of the claimed design. The broken lines throughout the drawing figures depict portions of the X-strap dog harness that form no part of the claimed design. The broken lines on the harness depict stitching and form no part of the claimed design.