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