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:

''"At last! At last! Huzzah! The plesiosaur is...complete! I never thought I would live to see this day! It's fantastic! Brilliant! A wonderful, joyful occasion! Delight ripples through my body and soul! But I'm getting carried away. Now then, where shall I begin with these leviathans? The great plesiosaurs... In actuality, plesiosaurs were not dinosaurs, but aquatic reptiles, which ruled the seas during the Mesozoic era. There were two main types of plesiosaurs, both of which had broad bodies, four large flippers, and short tails. The two groups were distinct in that one had long necks and small heads, the other short necks and large heads. The plesiosaurs ranged in size from 8 to 46 feet long, and their diets consisted of fish and other marine creatures. The first plesiosaur fossil was discovered in 1824, and subsequent findings have been made on every continent. Incredible, eh wot? Indeed! What I wouldn't do to dig one up myself! The thrill! Where in blazes is my shovel? Hoo hoo hoo! Oh, my! I must beg your pardon. The very idea of field work gets my crumpets toasting!"''

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

"This creature emerged toward the end of the era of the dinosaurs... To be accurate, it was not a dinosaur but rather a type of long-neck reptile. The first plesiosaurus fossil was found 35 years ago by a high-schooler! Amazing! I shan't tell you what MY greatest high-school accomplishment was... Wouldn't it be wonderful to discover a new fossil?! I hope to...someday."

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

"...Most excellent news! The plesiosaurus has been completed! Hoo my, such a staggering event! The plesiosaurus appeared toward the end of the dinosaurs' days on earth. It was a reptile that lived in the ocean, and it's called "the long-necked dinosaur" in Japanese. Indeed, someone named Suzuki found the first one and named it that... But you see, he was still a high-school student when he discovered the fossils. He quickly became the neighborhood hero, wot!"

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.