The Collaborative International Dictionary of English v.0.48:

gerenuk \ger"e*nuk\ (g[e^]r"[-e]*n[oo^]k), n.
   A slender East African antelope (Litocranius walleri) with
   a long slim neck and backward-curving horns; called also
   Waller's gazelle, and in German Giraffengazelle. It feeds
   on the foliage of bushes and trees, and often stands erect on
   its hind legs, leaning against the bush, to browse on the
   higher branches; in this habit it is distinctive and easy to
   recognize.

   Syn: Litocranius walleri.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

gerenuk
    n 1: slender East African antelope with slim neck and backward-
         curving horns [syn: gerenuk, Litocranius walleri]