Stegosaurus

The Stegosaur (or Stegosaurus in ) is a three-part fossil that can be donated to the museum in all games to date.

In Animal Crossing
"Finally! The day has come! Our stegosaurus is complete! Dare I believe it? Might this not be some cruel dream? Nay, it is true, indeed! I say to you: HOOOO! Spectacular! Superb! My whole being thrills! Now then, where to begin? Much has been written about the noble stegosaurus... It was the largest of the plated dinosaurs, and lived during the late Jurassic period, wot wot. It is famous for the two rows of alternating, bony plates surmounting its body and its four formidable tail spikes. Ka-smash! Yes, that's correct. It's a tail-spike chop! Oh... Please pardon me! I was overcome with emotion."

In
"It emerged right around the middle of the era when dinosaurs roamed the earth. The diamond-shaped plates embedded along its back are actually made of bone, wot? Indeed, they were used to keep it cool and also to thwart its enemies. Ah. Hoo. Indeed, so sorry. Stegosaurs never fail to set my beak blabbing!"

In City Folk
"...It is done! Hootie hoo, the stegosaurus is all done! Absolutely spectacular! The stegosaurus appeared in the middle of the days of the dinosaurs. Its defining characteristics were the plates extending out of its back. These were actually bones, eh wot? They were used to regulate the beastie's body temperature, much as an elephant uses its ears. Its brain was so small, if you stomped its tail, it would not feel it for some time! Or so they say. HOO! I...beg your pardon! I was so enthralled I...I spouted hot air like a boiling teapot, eh wot?"

In New Leaf
"The name stegosaurus literally means "roofed lizard" or "covered lizard." This name came from a belief at the time of discovery that it was covered in bony plates like a shell. We now know that the plates stood fully upright along its back and were covered in thick skin. Beyond protection from harm, they were likely used to regulate body temperature."

In New Horizons
"You can't talk about Stegosaurus without talking about the distinctive diamond-shaped plates on its back. These plates, while made of bone, were not actually connected to the animal's skeleton! They simply grew out from the skin, remarkably enough, and were up to two feet tall and similarly wide. It's not clear exactly how the plates were arranged or what they were for. Yet more mysteries of the ancients!"

In Real Life
Stegosaurus is a genus of herbivorous dinosaur. It lived in the Late Jurassic, 155 to 150 million years ago. They lived mostly in what is now the western United States, although one fragmentary specimen is known from Portugal. They could reach 9 meters in length and weight 5.3-7 tons. Based on the locations of their fossils, Stegosaurus likely preferred drier climates than many of its contemporaries, such as Diplodocus, Apatosaurus, and Allosaurus.

Along with Ankylosaurus, they are classified in the clade Thyreophora, a group of armored ornithischian dinosaurs. Stegosaurians are known for their modified osteoderms which grow along the back of the animals. Stegosaurus in particular has exceptionally large, plate-like osteoderms, unlike its more spiky relatives. It also has four spikes (arranged in two pairs) on its tail, in what is known as a "thagomizer". Stegosaurus likely used its thagomizer for defense, as some specimens of Allosaurus, the apex predator of the time, show wounds on their tails that match the shape of a thagomizer.