Class Mammalia Linnaeus, 1758 - mammals
   Subclass Theria Parker & Haswell, 1897
     Order Artiodactyla Owen, 1848 - even-toed ungulates see Order Cetartiodactyla Montgelard, Catzeflis & Douzery, 1997
       Family Hippopotamidae Gray, 1821 - hippopotamuses
         Genus Hexaprotodon Falconer & Cautley, 1836 [monotypic]
           1 ‡Hexaprotodon madagascariensis (Guldberg, 1883) see Hippopotamus madagascariensis Guldberg, 1883
           2 †[Hexaprotodon sivalensis] (Falconer & Cautley, 1836)
           3 Hexaprotodon liberiensis (Morton, 1849) - pygmy hippopotamus see Choeropsis liberiensis (Morton, 1844)