Styracosaurus

The Styracosaurus is a three-part fossil in and.

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

"...What, hoo?! It seems the styracosaurus is complete! Wondrous! Just wondrous! The styracosaurus appeared in the late days of the dinosaurs. Its characteristic horns and collar looked like frills—very dino-fashionable, wot wot! And actually, they had to be, because the rear frill was used by males to attract females. I guess male-female relations haven't changed much in all these years, eh wot? HOO! I...beg your pardon! I was so enthralled, I... I spouted hot air like a boiling teapot, eh wot?"

The Styracosaurus can be found in the first room of the fossil exhibit in the museum.

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

"Styracosaurus literally means "spiked lizard." It looked roughly like a more intense triceratops. In addition to the spike on its nose, it had three sets of horns extending from its gaudy neck frill. Fossils have generally been found in groups, so the belief is that it was a herd animal."

Real-world information
Styracosaurus albertensis is a herbivorous ceratopsian dinosaur that lived around 75 million years ago in modern-day Alberta, Canada. Its name, meaning "spiked lizard", comes from the exceptional number of spikes in its skull, including four to six spikes on its neck frill, a small jugal horn on each cheek, and a massive horn on its nose. It could reach 5.5 meters in length, stand 1.8 meters high, and weight 3 tons.