Dunkleosteus

The Dunkleosteus is a stand-alone fossil that can be donated to the museum in.

In New Horizons
"The Dunkleosteus flourished long before the dinosaurs and was a sort of armored fish. Curiously, only fossils for the head and shoulders have been found. We must simply imagine the rest! While its face was rather frightening, I like to picture a cute little tail and perhaps some fluffy paws on its fins. Such speculation is not scientific, of course, and essentially amounts to paleontological fan fiction. I own that. " —Blathers

In Real Life
Dunkleosteus is a placoderm, or armored fish, from the Late Devonian period. It was the largest placoderm known, with some species reaching nearly 9 meters and weighing 4 tons, making it comparable in size to a female orca and significantly larger than a great white shark. It possessed a powerful jaw, able to open and close within 60 milliseconds and exert a bite force of 6000-7400 N (comparable to that of a T. rex). This has led many to speculate that Dunkleosteus fed on ammonites and other placoderms.