Although the presence of 44 species of mammals are documented, American pronghorn, mule deer, jackrabbits, cotton-tail rabbits, least chipmunks, and Richardson ground squirrels are probably the only mammals the average visitor is likely to see during a casual summer visit. The majority of the mammals are small, active only at night, confined to small isolated habitats, or are so wary of humans that they are only rarely seen.
A few mule deer (Odocoileus hemionus) may reside on the monument throughout the year, but most migrate in late fall to winter range located elsewhere.
Pronghorn (Antilocapra americana) are usually somewhere on the monument from late spring through late fall or early winter. They also migrate to wintering areas outside the monument as snow accumulates.
Elk (Cervus elaphus) are seen on the monument occasionally in summer, but are more common in late fall and winter. Sizable herds of elk have spent at least part of the winter on the monument in recent years, and are frequently seen on the western and southern slopes of Fossil Butte and Cundick Ridge. Read Elk on the Landscape, and Observations of Elk Movement Patterns on Fossil Butte National Monument, a research project lead by the U.S. Geological Survey's Northern Rocky Mountain Science Center.
A few moose (Alces alces) are usually on or somewhere near the monument at all times of the year.