And the card says: True, he only sheds the outer shell.
A bit more info from
http://www.azantelope.org/Facts___Research/About_Pronghorn/about_pronghorn.html
Pronghorn have true horns, in that the horn sheath is composed of fused hairs which cover a bone core. The horn sheath is shed each year in October-November. In comparison, deer and elk have antlers which are composed entirely of bone and shed completely each year. Both sexes of pronghorn have horns, but those of the female are much smaller (4 inches), seldom exceeding the length of the ear. Horns on males reach their maximum length, 12-20 inches, by the beginning of breeding season, July or August.