The Collaborative International Dictionary of English v.0.48:

Shire horse \Shire horse\
   One of an English breed of heavy draft horses believed to be
   descended largely from the horses used in war in the days of
   heavy armor. They are the largest of the British draft
   breeds, and have long hair on the back of the cannons and
   fetlocks. Brown or bay with white on the face and legs is now
   the commonest color.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

shire horse
    n 1: British breed of large heavy draft horse [syn: shire,
         shire horse]