Monoclonius was Edward Drinker Cope's third named ceratopsian, after Agathaumas and Polyonax. Several fossils were found by Cope, assisted by a young Charles Hazelius Sternberg, in the summer of 1876 near the Judith River in Chouteau County, Montana, only about a hundred miles (some 150 km) from the site of the Battle of the Little Bighorn, fought that June. The finds did not represent a single, let alone articulated, skeleton, but came from different locations. Together they included elements of most parts of the animal (only the feet were entirely missing), including the base part of a long nasal horn, part of the skull frill, brow horns, three fused cervical vertebrae, a sacrum, a shoulder girdle, an ilium, an ischium, two thighbones, a shinbone, a fibula and parts of a forelimb. Just two weeks after leaving Montana, Cope hastily described and named these finds on 30 October 1876 as the type species Monoclonius crassus. The specific name means "the fat one" in Latin. Since the ceratopsians had not been recognised yet as a distinctive group, Cope was uncertain about much of the fossil material, not recognizing the nasal horn core, nor the brow horns, as part of a fossil horn. The skull frill he interpreted as an episternum, an ossified part of the breastbone, and the fused cervicals he assumed to be anterior dorsals.
