The Collaborative International Dictionary of English v.0.48:

hippo \hip"po\ n.
   Same as hippopotamus.

   Syn: hippopotamus, river horse, Hippopotamus amphibius.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Hippo
    n 1: an ancient Numidian town in northwestern Africa adjoining
         present-day Annaba in northeastern Algeria [syn: Hippo,
         Hippo Regius]
    2: massive thick-skinned herbivorous animal living in or around
       rivers of tropical Africa [syn: hippopotamus, hippo,
       river horse, Hippopotamus amphibius]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "hippo":
   Jumbo, blimp, dinosaur, elephant, fatty, hathi, heavy, heavyweight,
   hippopotamus, jumbo, leviathan, lump, mammoth, mastodon, monster,
   pachyderm, potbelly, rhino, rhinoceros, river horse, roly-poly,
   swagbelly, tub, tub of lard, tun, whale, woolly mammoth