The Collaborative International Dictionary of English v.0.48:

marco polo sheep \marco polo sheep\, marco polo's sheep \marco
polo's sheep\n.
   An Asiatic wild sheep (Ovis poli) with exceptionally large
   horns; sometimes considered a variety of the argali: Ovis
   ammon poli.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Argal \Ar"gal\, Argali \Ar"ga*li\, n. [Mongolian.] (Zool.)
   A species of wild sheep (Ovis ammon, or Ovis argali),
   remarkable for its large horns. It inhabits the mountains of
   Siberia and central Asia.
   [1913 Webster]

   Note: The bearded argali is the aoudad. See Aoudad. The
         name is also applied to the bighorn sheep of the Rocky
         Mountains. See Bighorn.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Ovis ammon
    n 1: wild sheep of semidesert regions in central Asia [syn:
         argali, argal, Ovis ammon]