The Collaborative International Dictionary of English v.0.48:

Muscovy duck \Mus"co*vy duck`\ [A corruption of musk duck.]
   (Zool.)
   A duck (Cairina moschata), larger than the common duck,
   often raised in poultry yards. Called also musk duck. It is
   native of tropical America, from Mexico to Southern Brazil.
   [1913 Webster]

WordNet (r) 3.0 (2006):

muscovy duck
    n 1: large crested wild duck of Central America and South
         America; widely domesticated [syn: muscovy duck, musk
         duck, Cairina moschata]