The Collaborative International Dictionary of English v.0.48:

feedbag \feedbag\, feed bag \feed bag\n.
   a canvas bag that is used to feed an animal (such as a horse
   or mule); animal feed such as oats are placed in the bag, and
   it is fastened at the top of the head, covering the muzzle.
   The animal may then eat the contents while thus wearing the
   feedbag.

   Syn: nosebag.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

feedbag
    n 1: a canvas bag that is used to feed an animal (such as a
         horse); covers the muzzle and fastens at the top of the
         head [syn: nosebag, feedbag]