The Collaborative International Dictionary of English v.0.48:

Water buck \Wa"ter buck`\ (Zool.)
   A large, heavy antelope (Kobus ellipsiprymnus) native of
   Central Africa. It frequents the banks of rivers and is a
   good swimmer. It has a white ring around the rump. Called
   also photomok, water antelope, and waterbok.
   [1913 Webster]

   Note: The name is also applied to other related species, as
         the leche (Kobus leche), which has similar habits.
         [1913 Webster]