The ornamental design for a "pen holder", as shown and described.
FIG. 1 is a front elevational view of a pen holder showing my new design;
FIG. 2 is a right side elevational view, the left side being a mirror image thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a front elevational view, the writing instrument depicted by means of broken lines being shown for illustrative purposes only and forming no part of the claimed design.