Brontosaurus?

Paleontologist Othniel Marsh described a dinosaur as Brontosaurus excelsus in 1879. Marsh also described another dinosaur as Apatosaurus ajax in 1877. The name Apatosaurus is therefore an earlier name and it has priority over Brontosaurus if these two species actually belong in the same genus.

In 1903, Elmer Riggs studied Apatosaurus ajax and he concludes that it is an immature specimen of a species that is in the same genus as Brontosaurus excelsus. Therefore he synonymized Brontosaurus and Apatosaurus, and since Apatosaurus is the earlier name, it has priority. However, Henry Osborn either rejected or was unaware of Riggs' publication, so he labeled the specimen in the American Museum of Natural History Brontosaurus excelsus instead of Apatosaurus excelsus. Since this museum is well known, most people think that Brontosaurus is the correct name. Both the public and the scientific community therefore used the name Brontosaurus for the next few decades, until someone rediscovered Riggs' publication and demanded that Brontosaurus should be renamed Apatosaurus, according to the rules of the International Commission of Zoological Nomenclature (ICZN).

The rules of the ICZN are not set in stone, as their purpose is to maintain taxonomic stability. In fact, the ICZN can be petitioned to reject an older name, if the newer name has been used widely, because a strict, unbending adherence to the rules will actually result in taxonomic instability. Stephen Jay Gould in fact made such an argument in his essay "Bully for Brontosaurus." Nevertheless, Gould's plea fell upon deaf ears and most people, both scientists and amateurs, were told that Brontosaurus is not the valid name. Instead Apatosaurus is.

Then in 2015, several scientists re-examined the available material and they concluded that Apatosaurus is actually different than Brontosaurus. There are therefore 2 different genera, instead of one. If we accept the new classification, Brontosaurus excelsus will be the correct name for one of the best known dinosaurs of all time and Apatosaurus ajax will be classified in its own genus. Of course, not all scientists agree with the new study, so they will continue to insist that Apatosaurus excelsus is the correct name. For the time being, then, we will see two different names being used. Neither one is necessarily more correct because there is no official list of scientific names. Therefore anyone can use either Brontossaurus excelsus or Apatosaurus excelsus.

Finally, there is no objective method to decide whether 2 species are similar enough to be included in the same genus or if they are different enough to be classified as different genera. Those who agree with the 2 genera arrangement will therefore use the name Bronotosaurus, but those who think that only a single genus should be recognized will only use the name Apatosaurus.

Brontosaurus and Apatosaurus are in the same classification "Apatosaurinae" so maybe that is where the confusion is coming from. I know it's not much but yeah hoped it helped :D