The Collaborative International Dictionary of English v.0.48:

Addax \Ad"dax\, n. [Native name.] (Zool.)
   One of the largest African antelopes
   (Hippotragusnasomaculatus or Oryx nasomaculatus).
   [1913 Webster]

   Note: It is now believed to be the Strepsiceros (twisted
         horn) of the ancients. By some it is thought to be the
         pygarg of the Bible.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Strepsiceros
    n 1: African antelopes: kudus; bongos; nyalas; bushbucks [syn:
         Tragelaphus, genus Tragelaphus, Strepsiceros, genus
         Strepsiceros]