 In 1888 and 1889 , Othniel Charles Marsh described the first well preserved horned dinosaurs , Ceratops and Triceratops . In 1890 Marsh classified them together in the family Ceratopsidae and the order Ceratopsia . This prompted Cope to <unk> his own specimens and to realized that Triceratops , Monoclonius , and Agathaumas all represented a single group of similar dinosaurs , which he named <unk> in 1891 . Cope <unk> Monoclonius as a horned dinosaur , with a large nasal horn and two smaller horns over the eyes , and a large frill . 
