The Collaborative International Dictionary of English v.0.48:

Capercailzie \Ca"per*cail`zie\, or Capercally \Ca"per*cal`ly\,
   n. [Gael, capulcoile.] (Zool.)
   A species of black Old World grouse (Tetrao uragallus) of
   large size and fine flavor, found in northern Europe and
   formerly in Scotland; -- called also cock of the woods and
   horse of the wood. [Written also capercaillie,
   capercaili.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

horse of the wood
    n 1: large black Old World grouse [syn: capercaillie,
         capercailzie, horse of the wood, Tetrao urogallus]