Private Joseph Fields saw several bighorn sheep near the Yellowstone River and picked up a curved horn to bring back to camp. A favorite of trophy hunters, bighorn sheep declined dramatically in the 19th century, from 200,000 to several thousand. Increasing populations of domestic sheep passed on psoroptic scabies, among other diseases, while exotic grasses and fire suppression changed their habitat. Reintroductions have helped some extirpated populations regain a foothold.

State species of concern; one subspecies is Federally endangered., and one is extinct.