12x12 multimedia collage and ink on wood panel. Hippo comes from the Ancient Greek word for river horse. Hippos are the third largest land mammals behind elephants and rhinos. They range from 3,000 to 10,000 pounds. Hippos cannot swim and are too dense to float. To move in water, they push off the bottom and walk on shallow river bottoms. They can hold their breath underwater for up to five minutes. Hippos are threatened by habitat destruction.