Monoclonius was named by Edward Drinker Cope in 1876. Later, much taxonomic confusion was caused by the discovery of Centrosaurus, a very similar genus of ceratopsian, that is known from much better remains. Today, typical Monoclonius specimens are usually believed to be juveniles or subadults, in many cases of other genera such as Centrosaurus. Those specimens that remain under the name Monoclonius are mostly too incomplete or immature to be confidently matched with adult specimens from the same time and place. This is especially true of the type species, Monoclonius crassus. Therefore, Monoclonius is now usually considered a nomen dubium, pending further study.