Illustration of Archaeotherium, a prehistoric entelodont (grey, centre) and two Hesperocyon prehistoric canids running. Hesperocyons were extinct prehistoric mammal lived from around 42 to 31 million years ago, during the Late Eocene and Early Oligocene. Archaeotherium were pig-like animals lived from around 38 to 25 million years ago, during the Eocene and Oligocene epochs, in what is now North America. Although, pig-like in appearance, Archaeotherium were more closely related to hippopotamuses and whales.