AGATHAUMAS

Discovered in 1872 by Fielding Bradford Meek and Henry Martyn Bannister while scouring the Lance Formation (then called the Laramie) for fossil shells during Ferdinand Hayden's Geological Survey of the territories, Agathaumas sylvestris—"equal in size to the largest known terrestrial saurians and mammals" and at that time the largest land animal known to have lived in North America—came as quite a surprise.

Its name, meaning "Great Wonder of the forest", was coined later the same year by an excited Edward Drinker Cope on the strength of a legless rear-end that he initially compared to Hadrosaurus and Dryptosaurus and then to Cetiosaurus. In fact, the only type of "saurian" he didn't compare it to was a ceratopsid (horn-faced dinosaur), and there's a simple enough reason for that: a horn-faced dinosaur had yet to be discovered.

Cope's soon-to-be bone wars nemesis, Othniel Charles Marsh of Yale University, was the first to realise horn-faced dinosaurs existed as he coined Ceratops horridus for some skull fossils from the Laramie in 1888. That epiphany led to the realisation that his Ceratops montanus, found by John Bell Hatcher near Montana's Cow Creek the previous year, had a horned face too and that Bison alticornis, named the year before that, wasn't actually a huge Pliocene-aged Bison after all. Marsh now had enough relatives to raise Ceratopsidae to house these three strange beasts, but not without resistance. Citing the paucity of Ceratops remains and unconvinced they were any different to those he had named Agathaumas seventeen years earlier, Cope insisted on using Agathaumidae for this family.

Bison was shunted into Ceratops later in 1889 while Ceratops horridus became the holotype of Triceratops, and based on the size and shape of Agathaumas, it's probably Triceratops too. Strictly speaking, Cope's critter should have had priority name-wise, but there are three reasons why this was never going to happen; (1) we can't be 100% certain that Agathaumas and Triceratops are the same, (2) you can't replace the holotype of a horn-faced critter with a specimen that doesn't actually have a face, never mind a face with horns on it, and (3) the definitive 1907 monograph on horned dinosaurs was written by Marsh and his buddies from Yale. Old Cope was on a hiding to nothing from the jump, and his Agathaumas is little more than a historical curiosity. But he had the last laugh in 1897 when supervising paleoartist Charles Robert Knight during his now-famous painting of a horned and armoured critter for an article in The Century Magazine that he called "Agathaumas sphenocerus", based on several parts of several dinosaurs, none of which were Agathaumas.