The Collaborative International Dictionary of English v.0.48:

Hippopotamus \Hip`po*pot"a*mus\, n.; pl. E. Hippopotamuses, L.
   Hippopotami. [L., from Gr.?; "i`ppos horse + ? river. Cf.
   Equine.] (Zool.)
   A large, amphibious, herbivorous mammal (Hippopotamus
   amphibius), common in the rivers of tropical Africa. It is
   allied to the hogs, and has a very thick, naked skin, a thick
   and square head, a very large muzzle, small eyes and ears,
   thick and heavy body, and short legs. It is supposed to be
   the behemoth of the Bible. Called also zeekoe, and river
   horse. A smaller species (Hippopotamus Liberiencis)
   inhabits Western Africa.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

pachyderm \pach"y*derm\ (p[a^]k"[i^]*d[~e]rm), n. [Cf. F.
   pachyderme.] (Zool.)
   Any of various nonruminant hoofed mammals having very thick
   skin, including the elephant, rhinoceros, and
   hippopotamus, one of the Pachydermata.
   [WordNet 1.5 + 1913 Webster]

WordNet (r) 3.0 (2006):

hippopotamus
    n 1: massive thick-skinned herbivorous animal living in or
         around rivers of tropical Africa [syn: hippopotamus,
         hippo, river horse, Hippopotamus amphibius]