The ornamental design for a ponytailholderextension, as shown and described.
FIG. 1 is a perspective front view of a ponytail holder extension;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof; the right side elevation view being a mirror image thereof;
FIG. 5 is a top elevation view thereof; and,
FIG. 6 is a bottom elevation view thereof.
The broken lines shown in the drawings that illustrate portions of a ponytail holder and additional ponytail holder extensions form no part of the claimed design.