Myllokunmingia

The Myllokunmingia is a fossil that can be donated to the museum in.

In
When donating to the museum, Blathers will say the following:

"The myllokunmingia! Said to be one of the oldest fish-like animals, it lived well before the dinosaurs. It was about an inch long and seems to have had no lower jaw. It is awe-inspiring imagining such ancient life! There is much we still don't know. We may yet discover even older organisms with even fewer jaws!"

Real-world information
Myllokunmingia is one of the earliest chordates found. It lived in what is now southern China from 535 to 520 million years ago. Only one specimen is known, measuring 28 mm long and 6 mm high. It is hypothesized to be a vertebrate, although this has not been firmly established.