Acanthostega

Acanthostega is a standalone fossil: one piece completes it. Dig up fossils at the cracks on the ground, have Blathers assess them, then donate it to the museum or sell spares for 2,000 Bells.

Acanthostega lived around 365 million years ago and was one of the earliest animals with four limbs. It had eight digits on each front foot, but it still had gills and spent most of its life in water.