A distinct subspecies of pygmy hippopotamus existed in Nigeria until at least the 20th century, though the validity of this has been questioned. The existence of the subspecies, makes "Choeropsis liberiensis liberiensis" (or "Hexaprotodon liberiensis liberiensis" under the old classification) the full trinomial nomenclature for the Liberian pygmy hippopotamus. The Nigerian pygmy hippopotamus was never studied in the wild and never captured. All research and all zoo specimens are the Liberian subspecies. The Nigerian subspecies is classified as "C. liberiensis heslopi".