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]

WordNet (r) 3.0 (2006):

Marco Polo sheep
    n 1: Asiatic wild sheep with exceptionally large horns;
         sometimes considered a variety of the argali (or Ovis
         ammon) [syn: Marco Polo sheep, Marco Polo's sheep,
         Ovis poli]