The term 'River Horse' is a common name for the HIPPO (5 letters) and HIPPOPOTAMUS (12 letters). The HIPPOPOTAMUS is a large, semi-aquatic mammal native to sub-Saharan Africa. It is the third largest land mammal after the elephant and rhinoceros. It is an herbivore and spends most of its time in the water, where it is well adapted for swimming. Its skin is thick and its eyes, ears, and nostrils are located on the top of its head, allowing it to remain mostly submerged while still being able to breathe.