The ornamental design for a pethighchair, as shown and described.
FIG. 1 is a top front perspective view of the pet highchair;
FIG. 2 is a front elevation view of the pet highchair, with the rear elevation view being identical to the front view shown;
FIG. 3 is a right side elevation view of the pet highchair, with the left side elevation view being identical to the front view shown;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines shown in the drawings depict environmental subject matter and form no part of the claimed design.
The pet highchair is shown with a line-dot-line broken line which represents a symbolic break in its height. The appearance of any portion of the article between the break lines forms no part of the claimed design.