Ah, the majestic hippopotamus. The herbivorous mammals weigh between one-and-a-half and four tons, and they can grow up to 14 feet long. Nicknamed "river horses" by ancient Greeks, the animals spend up to 16 hours each day lumbering along the bottoms of rivers and lakes in sub-Saharan Africa, with only their eyes and nostrils bobbing above the surf.

Although they primarily only eat plants—munching on about 80 pounds of grass each day—hippos are one of the most aggressive animals on Earth. They can snap a canoe in half with their powerful jaws, and they kill about 500 people in Africa each year.

But this species is vulnerable and their numbers are decreasing. They're threatened by habitat loss and illegal hunting,