A fish about to be possessed.
Hagfish, the clade Myxini (also known as Hyperotreti), are eel-shaped slime-producing marine animals (occasionally called slime eels). They are the only known living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils; hagfish are basal to vertebrates, and living hagfish remain similar to hagfish 300 million years ago.
Like other animals, Hagfish are capable of being possessed. They can be found in the waterways of Dunwall, and some places, such as the Golden Cat, keep ornamental fish.
When swimming in water, hagfish will bite Corvo Attano, if he strays too close to one. Additionally, any unconscious or dead bodies thrown into waters containing hagfish will be eaten.
Fish are apparently eaten by the people of Dunwall, as you can consume cans of Jellied eels and hagfish. There are also some extremely large, but unnamed fish being served to the guests at "Lady Boyle's Last Party".