👉 Hippopotamus is a species of aquatic mammal that lives in rivers, lakes, and oceans around the world. They are known for their large size, long necks, and ability to swim upstream or downstream at speeds of up to 40 miles per hour (65 km/h). Hippopotamuses are highly social animals and have a complex diet consisting of aquatic plants, fish, crustaceans, and small mammals like frogs.