Class Mammalia Linnaeus, 1758 - mammals Order Artiodactyla Owen, 1848 - even-toed ungulates see Order Cetartiodactyla Montgelard, Catzeflis & Douzery, 1997 Family Hippopotamidae Gray, 1821 - hippopotamuses Genus Hexaprotodon Falconer & Cautley, 1836 Hexaprotodon liberiensis (Morton, 1849) - pygmy hippopotamus see Choeropsis liberiensis (Morton, 1844)