The Collaborative International Dictionary of English v.0.48:

Water deer \Wa"ter deer`\ (Zool.)
   (a) A small Chinese deer (Hydropotes inermis). Both sexes
       are destitute of antlers, but the male has large,
       descending canine tusks.
   (b) The water chevrotain.
       [1913 Webster]

WordNet (r) 3.0 (2006):

water deer
    n 1: largest chevrotain; of marshy areas of west Africa [syn:
         water chevrotain, water deer, Hyemoschus aquaticus]