Monoclonius, meaning “single stem”, was a ceratopsian dinosaur from the Late Cretaceous Period. It was discovered at the Judith River Formation in Montana. The name monoclonius is derived from the single root system of its teeth. Monoclonius was first described by Edward Drinker Cope in 1876. It was discovered only 100 miles from the Battle of the Little Bighorn which occurred that same year. Most of the skeleton was recovered, except for the feet. At the time not much was known about ceratopsians, and Cope was unable to recognize that the horn core was part of the fossil horn. He later was able to determine that monoclonius was part of a larger group of ceratopsian dinosaurs due to other skeletal dinosaurs which were found and described by other paleontologists.

Monoclonius has a large nasal horn and two smaller horns over the eyes. It has a large frill with broad openings. It was an herbivorous dinosaur with a parrot-like beak . It was able to chew through tough plant matter with its strong jaw.

Monoclonius has been picked apart in detail over the years by several notable paleontologists and scientists trying to decipher the true genus of the animal. Eventually Monoclonius was given correct status, although very similar to centrosaurus, they are sometimes considered conspecific. The species names given to monoclonius include M. dawsoni, M. crassus, M. flexus, M. nasicornis, and M. cutleri.