The most important underwater being for the Ojibwa is Mishipeshu, which means "the Great Lynx." This fantastic dragon-like animal resembles a feline with horns, symbols of his power. It has palmed paws that enable him to swim fast, and his back and tail are covered with scales. Mishipeshu lives in the depths of big lakes. Although he has a feline shape and is an amphibian, he is always described as a reptile.

He is feared by all Ojibwa because he is the cause of waves, rapids and whirlpools, and he even breaks the ice in winter, thus claiming numerous victims. In the area of Churchill River, there used to be a game called "Mishipeshu" that symbolized this being's drowning power. A child, randomly selected, held the role of the aquatic monster; he had to catch his friends and throw them into the water.

All lakes can be inhabited by one of these beings, but some are known to change their appearance abruptly (eg, sudden strong wind or fog), to contain deep depressions, or to have a particular coloration; these are all signs of the Great Lynx's presence.

It is sometimes said that the horns and scales of Mishipeshu are made of pure copper, and most Indigenous populations in the Great Lakes see those aquatic monsters as the guardians of this metal. During a fight with a Mishipeshu, a youth reported that he broke a part of the monster's tail when hitting it with a paddle. This fragment was made of copper and he kept it to acquire luck at hunting and fishing.

Although Mishipeshu and the other underwater beings are essentially considered harmful, they were also necessary to the Algonquians because they gave them protection and medicine. The Horned Snakes and the underwater Lynx were particularly dangerous, but they also secured successful hunts and an abundance of food. It was also possible to call upon their power to secure a good fish catch, although their help was most precious for some healings. Medicine bundles were made of snake skin. These beings were also the guardians of the midewigun, the Midewiwin lodge. Moreover, the term "Mishipeshu" was used as a family name, showing that this being was not only seen as harmful.

Although the most important enemy of the Mishipeshu and other underworld reptiles was the Thunderbird, these also had to face the destruction programmed by Nanabozo in the myth that tells how the world was destroyed by a catastrophic flood.