The ornamental design for a ponytail accessory, as shown and described.
FIG. 1 is a rear view of the ponytail accessory.
FIG. 2 is a front view of the claimed design.
FIG. 3 is a left side view of the claimed design.
FIG. 4 is a right side view of the claimed design.
FIG. 5 is a top view of the claimed design.
FIG. 6 is a bottom view of the claimed design; and,
FIG. 7 is a perspective view of the claimed design being worn as an attachment to hair of an end user that is depicted in broken lines forming no part of the claimed design.