Sapo is one of the best places in the world — and effectively the most accessible — to be in the same forest as the pygmy hippopotamus (Choeropsis liberiensis). They are nocturnal, solitary, and shy, so actually seeing one is rare even for researchers; tracks, wallows and droppings along the river edges are much more common, and ranger-guided early-morning walks along the rivers will sometimes turn up signs.
Camera-trap studies in the park since the 2000s have confirmed a viable population, although exact numbers are uncertain. Habitat protection inside the park is the only thing that has kept the species from sliding further; outside Sapo the species is heavily threatened by logging, hunting and habitat loss across its tiny range.