The ornamental design for a petfountain, as shown and described.
FIG. 1 is a top, front and left side isometric view of a pet fountain showing our new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is bottom plan view thereof.
The broken lines illustrate portions of the pet fountain that form no part of the claimed design. The dash-dot broken lines represent the bounds of the claim and form no part of the claimed design.