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]