The Mi-ni-wa-tu, or “sea monster”, is known from the folklore of the Tetons, and may be found in the Missouri River.
A mi-ni-wa-tu is an amphibious creature with a body like that of a buffalo, and covered with red hair. It has a single horn in its forehead, and a single eye. Its back is notched like a saw or gear.
The mi-ni-wa-tu may be safely seen at night when it swims powerfully up the river, churning the water and glowing like fire; in the spring, it breaks up river ice. Seeing a mi-ni-wa-tu during the day causes confusion and loss of sight. Madness sets in; after a day of convulsions, the unfortunate victim dies.
Dorsey, J. O. (1889) Teton Folklore Notes. Journal of American Folklore, Vol. II, No. IV, pp. 133-139.