The Collaborative International Dictionary of English v.0.48:

Greylag \Grey"lag`\, n. (Zool.)
   See Graylag.
   [1913 Webster]

WordNet (r) 3.0 (2006):

greylag
    n 1: common grey wild goose of Europe; ancestor of many domestic
         breeds [syn: greylag, graylag, greylag goose,
         graylag goose, Anser anser]