Shellwhale

The Shellwhale (貝クジラ, Kai Kujira) is an animal whose body can be found in the waters of Shellwhale Cove. While asleep, Gamakatsu mentions that he can hear its cries for help in his dreams. He also refers to it as the "Big Catch".

Characteristics
The Shellwhale is a large, bony white whale. Its large head resembles a clam shell and it has a pronounced underbite. There are dark holes for its eyes, to further give it a skeletal appearance.

Hint
It seems as though something can be heard in the waters of Shellwhale Cove, there must be a way to find out what it is.