The animal that's name means "river horse" in Greek is the hippopotamus. The word "hippopotamus" comes from the ancient Greek words "hippos" which means "horse" and "potamos" which means "river", so together they mean "river horse". Despite its name, however, the hippopotamus is more closely related to pigs and whales than to horses.

The term "hippopotamus" indeed has its roots in ancient Greek, where "hippos" means "horse" and "potamos" means "river." Thus, when combined, it translates to "river horse." Despite the name suggesting a relation to horses, the hippopotamus is, in fact, more closely related to pigs and whales. This intriguing naming reflects the ancient Greek understanding of the animal's habitat near rivers and its large, bulky appearance, which might have drawn a parallel with a horse-like creature residing in the water.