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:

River \Riv"er\, n. [F. riv[`e]re a river, LL. riparia river,
   bank of a river, fr. L. riparius belonging to a bank or
   shore, fr. ripa a bank or shore; of uncertain origin. Cf.
   Arrive, Riparian.]
   1. A large stream of water flowing in a bed or channel and
      emptying into the ocean, a sea, a lake, or another stream;
      a stream larger than a rivulet or brook.
      [1913 Webster]

            Transparent and sparkling rivers, from which it is
            delightful to drink as they flow.     --Macaulay.
      [1913 Webster]

   2. Fig.: A large stream; copious flow; abundance; as, rivers
      of blood; rivers of oil.
      [1913 Webster]

   River chub (Zool.), the hornyhead and allied species of
      fresh-water fishes.

   River crab (Zool.), any species of fresh-water crabs of the
      genus Thelphusa, as Thelphusa depressa of Southern
      Europe.

   River dragon, a crocodile; -- applied by Milton to the king
      of Egypt.

   River driver, a lumberman who drives or conducts logs down
      rivers. --Bartlett.

   River duck (Zool.), any species of duck belonging to
      Anas, Spatula, and allied genera, in which the hind
      toe is destitute of a membranous lobe, as in the mallard
      and pintail; -- opposed to sea duck.

   River god, a deity supposed to preside over a river as its
      tutelary divinity.

   River herring (Zool.), an alewife.

   River hog. (Zool.)
      (a) Any species of African wild hogs of the genus
          Potamoch[oe]rus. They frequent wet places along the
          rivers.
      (b) The capybara.

   River horse (Zool.), the hippopotamus.

   River jack (Zool.), an African puff adder (Clotho
      nasicornis) having a spine on the nose.

   River limpet (Zool.), a fresh-water, air-breathing mollusk
      of the genus Ancylus, having a limpet-shaped shell.

   River pirate (Zool.), the pike.

   River snail (Zool.), any species of fresh-water gastropods
      of Paludina, Melontho, and allied genera. See Pond
      snail, under Pond.

   River tortoise (Zool.), any one of numerous fresh-water
      tortoises inhabiting rivers, especially those of the genus
      Trionyx and allied genera. See Trionyx.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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