Eusthenopteron

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

In New Horizons
"The Eusthenopteron is famous for being the link between fish and land animals long before dinosaurs. It seems to have strong fins capable of pulling it around areas where the water was shallow. When most creatures lived in the sea... they dreamed of land. If not for them, we mightn't be here today! Imagine if we'd not left the oceans... How might fashion and music be different in an aquatic world? How would we resolve differences? Perhaps some sort of ink-squirting contest of champions? " —Blathers

In Real Life
Eusthenopteron was a sarcopterygian (lobe-finned fish) from the Late Devonian. It is a tetrapodomorph, making it more closely related to modern tetrapods (land vertebrates) than to modern lobe-finned fishes like lungfishes and coelacanths. While very much aquatic (unlike later tetrapodomorphs like Acanthostega), it nevertheless exhibits anatomical features exclusive to tetrapodomorphs (including tetrapods), including choana, or internal nostrils, and possibly bone marrow.