The word "hippopotamus" derives from the Greek hippos, "horse", and potamos, "river", meaning "horse of the river". The hippo was well known to the Greeks and Romans as the fierce Beast of the Nile. In Egyptian mythology, the hippopotamus-headed Tawaret was a goddess of protection in pregnancy and childbirth, because of the protective nature of a female hippopotamus toward her young. The Greek historian Herodotus described the hippo in The Histories (c. 440 BC) and the Roman historian Pliny the Elder wrote about the hippopotamus in his encyclopedia Naturalis Historia (c. 77 AD). On Roman coins the hippopotamus was sometimes used as a reverse type to commemorate games at which they were displayed.