Dimetrodon

The Dimetrodon is a multi-part fossil in the introduced in. In games prior to it has three parts, and in  it has two parts.

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

"Despite its reptilian appearance, the dimetrodon was not actually a dinosaur, as you'd assume. It was, in fact, a mammal-like reptile that went extinct before the first dinosaurs appeared. It had knife-like teeth for slicing as well as smaller ones for crushing. The large fan on its back was used to regulate body temperature-- an important feature of the time."

Real-world information
Dimetrodon, meaning "two measures of teeth", is an extinct genus of non-mammalian synapsids that lived during the Cisuralian (Early Permian), around 295–272 million years ago (Ma). It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, fossils have been found in Germany. Over a dozen species have been named since the genus was first described in 1878.

Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs. Reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than to modern reptiles, though it is not a direct ancestor of mammals. Dimetrodon is assigned to the "non-mammalian synapsids", a group traditionally called "mammal-like reptiles". This groups Dimetrodon together with mammals in a clade (evolutionary group) called Synapsida, while placing dinosaurs, reptiles and birds in a separate clade, Sauropsida. Single openings in the skull behind each eye, known as temporal fenestrae, and other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids. It was a carnivore and had a fan to regulate body temperature.