The ornamental design for an arrowhead, as shown and described.
FIG. 1 is a front elevation view of an arrowhead.
FIG. 2 is a rear elevation view of the arrowhead.
FIG. 3 is a left side elevation view of the arrowhead.
FIG. 4 is a right side elevation view of the arrowhead.
FIG. 5 is a top view of the arrowhead.
FIG. 6 is a bottom view of the arrowhead.
FIG. 7 is a left front isometric view of the arrowhead; and,
FIG. 8 is a left rear isometric view of the arrowhead.
The broken lines shown in the drawings that are immediately adjacent to the shaded areas and define unshaded areas, represent bounds of the claimed design. All other broken lines are directed to environment. None of the broken lines in the drawings form any part of the claimed design.