what is the pygmy hippopotamus?
It  is a small hippopotamid.
Pygmy hippopotamus
is a small hippopotamid 
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Pygmy hippopotamus [SEP] Introduction
The pygmy hippopotamus ("Choeropsis liberiensis" or "Hexaprotodon liberiensis") is a small hippopotamid which is native to the forests and swamps of West Africa, primarily in Liberia, with small populations in Sierra Leone, Guinea, and Ivory Coast. The pygmy hippo is reclusive and nocturnal. It is one of only two extant species in the family Hippopotamidae, the other being its much larger relative, the common hippopotamus ("Hippopotamus amphibius") or Nile hippopotamus. The pygmy hippopotamus displays many terrestrial adaptations, but like the hippo, it is semiaquatic and relies on water to keep its skin moist and its body temperature cool. Behaviors such as mating and giving birth may occur in water or on land.