Monoclonius (meaning "single sprout") is an extinct genus of herbivorous ceratopsian dinosaur found in the Late Cretaceous layers of the Judith River Formation in Montana, United States, and the uppermost rock layers of the Dinosaur Park Formation in Alberta, Canada dated to between 75 and 74.6 million years ago.

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.

Just two weeks after leaving Montana, Cope hastily described and named these finds on 30 October 1876 as the type species Monoclonius crassus.

The skull frill he interpreted as an episternum, an ossified part of the breastbone, and the fused cervicals he assumed to be anterior dorsals.

[5] The second was Monoclonius sphenocerus, the "wedge-horned" from Greek σΦηνός, sphènos, "wedge", based on specimen AMNH 3989, a 325 millimetres (12.8 in; 1.066 ft) long nasal horn, found by Sternberg in 1876 on Cow Island in the Missouri.

[6] In 1895, for financial reasons, Cope was forced to sell a large part of his collection to the American Museum of Natural History.

Hatcher reexamined the presumed type specimen of M. crassus and concluded it in fact represented several individual animals and thus was a series of syntypes.

Therefore, he selected one of these as the lectotype, the name-bearing fossil, and chose the distinctive left parietal, forming the dorsal part of the neck frill.

The several squamosals, sides of the frill, in the collection could not be associated to this lectotype and he did not believe that Cope's orbital horn (catalogued under a different number) belonged to it.

[7] In the years after Cope's 1889 paper, it appears that there was a tendency to describe any ceratopsid material from the Judith River beds as Monoclonius.

Comparing the parietals of Monoclonius and Centrosaurus, he concluded that any differences were caused by the fact that the M. crassus lectotype had been that of an old animal and damaged by erosion.

[11] The next round fell in 1917 to Brown in a paper on Albertan centrosaurines, which, for the first time, analyzed a complete ceratopsian skeleton, specimen AMNH 5351 found by him in 1914, which he named Monoclonius nasicornus ("with the nose-horn").

In the same paper he described yet another species, Monoclonius cutleri, the epithet honouring William Edmund Cutler, based on specimen AMNH 5427, a headless skeleton featuring skin impressions.

Lull described another almost complete specimen from Alberta: AMNH 5341, presently exhibited as YPM 2015 at Yale's Peabody Museum in an unusual way: the left half shows the skeleton, but the right side is a reconstruction of the living animal, and referred it to a Monoclonius (Centrosaurus) flexus.

The last position was from 1990 defended by Peter Dodson who claimed that specimen AMNH 3998, the M. crassus lectotype, differed from the Centrosaurus apertus holotype in having a very thin parietal close to the skull frill edge.

[21] In 1998 Dodson and Allison Tumarkin argued that the bone structure could also be explained by species-specific pedomorphosis, the retention by adults of juvenile traits.

[22] However, in 2006 Michael Ryan concluded that the M. lowei holotype was an exceptionally large subadult after all, as shown by a third epiparietal, osteoderm on the frill edge, just beginning to develop, and skull sutures which are not completely closed.

At the time, Monoclonius, Agathaumas, and Triceratops were all thought to be close relatives that differed mainly in the arrangement of the horns and the presence of openings in the frill.

On April 6, 2011, the Tippett Studio had published on its YouTube official channel a digital restoration of the Prehistoric Beast short.