Answer: Hippos

Hippopotamuses seem to get around just fine in water, despite their hulking and (frankly) ungraceful bodies. Surely they must be incredibly powerful swimmers to propel themselves so quickly and efficiently through the water, right? Surprisingly, hippos can’t actually swim. They’re simply too dense for the activity, and are actually in a sort of state of controlled sinking at all times.

What you’re witnessing when you see what appears to be a hippo gliding through the water is actually the hippo walking “through” shallow water. When the water is deeper than they are tall, they resort to a powerful porpoise-like leaping motion. Frankly, in our opinion, that makes hippos all the more terrifying—to be strong enough to propel your 2,870-3,310 pound (1,300-1,500 kilogram) body through the water simply by bounding along the bottom is to be incredibly strong indeed.

It would seem rather appropriate then, given their strength and method of locomotion in the water, that the Ancient Greeks called them “river horses” (“horse of the river”) and our modern name for them is derived, in turn, from that.