We claim, the ornamental design for a pet urn, as shown and described.
FIG. 1 is a perspective view of a pet urn, showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view;
FIG. 4 is a side view;
FIG. 5 is a top view; and,
FIG. 6 is a bottom view.
The dot-dash line depicts the bounds of the claim and forms no part of the claimed design. All other broken lines in the drawing depict portions of the pet urn that form no part of the claimed design.