The gigantic, herbivorous Brontosaurus, which features an iconically long neck and tail, may be one of history's most famous dinosaurs, but its existence as a unique species was debunked in 1903, when someone realized that the skeleton that had been labeled as a Brontosaurus was actually an Apatosaurus. In this sense, the Brontosaurus was not renamed but rather determined to have never existed as a separate species. The same man, a Yale paleontologist named Othniel Charles (O.C.) Marsh, named both the Apatosaurus and the Brontosaurus, with the Apatosaurus discovery predating the Brontosaurus naming by two years.
When O.C. Marsh first named the Apatosaurus in 1877, he was working with relatively limited fossil evidence, which is likely the reason why he didn't recognize the same anatomy when it was time to name the nearly complete skeleton that came to be known as Brontosaurus two years later. There may have been other motivations behind this rush to name two different species, including a supposedly bitter rivalry between Marsh and another paleontologist named Edward Drinker Cope. In the end, the truth came to light, and though the animal named Brontosaurus was more complete and more famous, the Apatosaurus name came first, giving it scientific precedence.