Plesiosaurus

The Plesiosaurus (known as the Plesiosaur prior to ) is a three-part fossil in the introduced in  that appears in all subsequent games to date.

In
When donating the final part to Blathers in, he will provide the following information about the fossil:

In
When donating to Blathers in, he will provide the following information about the fossil:

In
When donating the final part to Blathers in, he will provide the following information about the fossil:

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

"While not a dinosaur itself, the plesiosaur, a marine reptile, did live alongside the dinosaurs. These creatures could reach lengths of 23 feet or longer, the bulk of that being their long necks. Plesiosaurs are commonly mistaken for certain kinds of mythical sea monsters, such as "Nessie.""

Real-world information
Plesiosaurs were a group of Mesozoic marine reptiles. They first evolved 203 million years ago, and there is evidence that they diversified rapidly in the next 2 million years, even as other animals were wiped out by the Triassic-Jurassic extinction event. They remained diverse throughout the Jurassic and Cretaceous, and could be found in oceans worldwide. They went extinct 66 million years ago, in the same extinction event that killed the non-avian dinosaurs.

The specific genus of plesiosaur in-game is identified as Futabasaurus. This species lived around 85 million years ago in modern-day Japan. It is a member of the elasmosaurids, plesiosaurs with unusually long necks (with some species having over 70 neck vertebrae). Futabasaurus has been estimated to reach 6.4-9.2 meters and weigh 3-4 tonnes. Based on the bones in the type specimen, it is likely that they were hunted by sharks.