In 1866, back when the scientific study of dinosaurs was only just beginning in North America, the naturalist E.D. Cope received word that workers at the West Jersey Marl Company in Gloucester County, New Jersey, had discovered the gigantic bones of an unknown fossil animal. As Cope did much of his work just across the Delaware River in Philadelphia, he did not have far to travel, and upon arriving at the site he was able to collect elements of the jaws, skull, legs, hip, tail and arm (including an immense claw) from the chocolate-colored Cretaceous marl. Altogether this material would come to represent a dinosaur Cope named Laelaps aquilunguis, named after the dog of Greek mythology which never failed to catch its prey (Laelaps) and for the large talon found among the remains (aquilunguis, meaning "eagle-clawed"). As he declared in its description, Cope believed it was "the most formidable type of rapacious terrestrial vertebrata of which we have any knowledge."
Cope was enthralled by this discovery. It was the first partial skeleton of a predatory dinosaur found in the United States, and the fact that its arms were clearly shorter than its hindlimbs caused paleontologists to reconsider what dinosaurs looked like. Rather than being the squat, crocodile-like monsters that the English anatomist Richard Owen envisioned, dinosaurs were cast as being more bird-like in form and habit, and in his scientific work Cope himself appears to have taken great joy in envisioned Laelaps leaping upon its prey, tearing at the hide of Hadrosaurus (also discovered in New Jersey) and shattering the armor of ancient crocodiles. Today we know the dinosaur as a tyrannosauroid closely related to the recently-discovered Appalachiosaurus, and though hypotheses of what it looked like have changed a bit since the late 19th century Cope's restorations of Laelaps was still pretty close to reality.
Yet Laelaps was not to keep its name. Unbeknown to Cope, that genus name had already been applied to a kind of mite, and this provided an opportunity for his rival, O.C. Marsh, to upstage him. Though they struck up a brief friendship upon meeting in Europe early in their career, back in the United States the two paleontologists quickly became fierce rivals, and the competition between them erupted into the famous "Bone Wars" of the late 19th century. Both in the field and in academic journals, both men vied for the unofficial title of "America's greatest paleontologist", and in the case of New Jersey's predatory dinosaur Marsh had found a way to rename one of Cope's most favorite discoveries.
In 1877, eleven years after it was first announced, Marsh renamed Cope's dinosaur Dryptosaurus ("tearing reptile") in a footnote of a description of another dinosaur, "Titanosaurus" (which, oddly enough, had already been used for another dinosaur, causing Marsh to later rename it Atlantosaurus). This must have been doubly frustrating for Cope. Not only had his "Laelaps" been renamed, but his rival had done so as an academic aside. While it is certainly true that the methods of writing scientific papers and descriptions have changed since 1877, I can't help but wonder if Marsh intentionally renamed "Laelaps" in a footnote specifically to irk Cope. (Not surprisingly given his stubborn temperament, Cope continued to use the name "Laelaps" for the rest of his career.)
This was not the only time that a prehistoric creature had to be renamed because the name it had been given was preoccupied. Another fossil creature from New Jersey, a crocodile originally named Holops, had to have its name changed to Holopsisuchus because its original name had already been given to an insect. Even more recently, it was discovered that the horned dinosaurs Diceratops and Microceratops needed to be renamed for much the same reason. Today they are known as Diceratus and Microceratus, respectively. Such conflicts are inevitable, especially as biologists of varying fields continue to catalog new genera, and while I must admit I like some of the original genus names, we all have to play by the rules when it comes to science.