Apatosaurus

The Apatosaurus (known as the Apatosaur in and ) is a three-part fossil in the  introduced in  that appears in all subsequent games except.

In
After donating the final part of the fossil in, its plaque in the museum will read:

"With its impossibly long neck and tail, many questions exist regarding the lifestyle of the apatosaurus. Herbivorous dinosaurs tended to have larger digestive tracts than carnivorous dinosaurs. This distinction is what usually accounted for the thicker body, which the apatosaurus had. You might hear it called a brontosaurus, though this name is scientifically redundant."

Trivia

 * In all games this fossil appears in, Blathers references that Brontosaurus excelsus, first described in 1879, was reclassified in 1903 as an Apatosaurus due to its similarities with earlier specimens. However, in 2015, new studies seemed to find enough differences between the two and suggested Brontosaurus be reconsidered a valid genus.

Real-world information
Apatosaurus was a herbivorous sauropod dinosaur that lived in North America during the Late Jurassic period. It reached over 20 meters long and weighed around 20 tons, although some specimens have been estimated to be even larger. Despite its size, its diet appears to have consisted primarily of ground-level plants.