The ornamental design for a mannequin, as shown and described.
FIG. 1 is a front view thereof,
FIG. 2 is a rear view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view thereof.
In the drawings, the broken lines of the stand illustrate environment, and the broken lines of the curved slit and rectangular box of FIG. 2 illustrate unclaimed subject matter, and form no part of the claimed design.