Piranu

The Piranu is an aggressive and territorial form of monstrous fish from Argentina. They have large black fish-like bodies with the heads of horses and large eyes. They live in deep freshwater rivers, and ram any boats which gets too close.

Citations:

Bane, Theresa. Encyclopedia of Beasts and Monsters in Myth, Legend and Folklore. United States, McFarland, Incorporated, Publishers, 2016.