Acanthostega

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

In New Horizons
"The acanthostega! Said to be one of the earliest amphibians, it existed well before dinosaurs. Because they lived as fish not long before, they still had gills and very webbed "hands." To toss away the life they knew and venture onto unknown lands... they must have been very brave! Hmm... Does it still count as bravery if you have no understanding of what you're doing?" –Blathers

In Real Life
Acanthostega gunnari was an animal from the late Devonian period and is representative of the evolution of tetrapods, or land vertebrates. While not well suited for life on land, it nevertheless exhibits important evolutionary advances in that direction. Notably, its locomotion was powered by its back fins instead of its front.