Ichthyostega is a fish confirmed to appear in Additional Creatures: Endemics.
Ichthyostega, meaning "Fish Roof", is a species of stem-tetrapod that lived in Greenland during the Late Devonian, 360 million years ago. Ichthyostega is relevant as it represents a link between fish and amphibians/tetrapods. Despite the limbed appearance, ichthyostega is still a stem-tetrapod, meaning that it could still technically be considered a fish rather than an amphibian. While having limbs and a pelvis, it still had gills, a fish-like spine, and its limbs were extremely weak, to where they couldn't support for the body effectively. This meant that land locomotion was more akin to a seal than a salamander.
Ichthyostega is currently featured in the beta version of Additional Creatures: Endemics, whose progress to release is unknown.