Source : Webster's Revised Unabridged Dictionary (1913)

Pacer \Pa"cer\, n.
   One who, or that which, paces; especially, a horse that
   paces.

Source : WordNet®

pacer
     n 1: a horse used to set the pace in racing [syn: {pacemaker}, {pacesetter}]
     2: a horse trained to a special gait in which both feet on one
        side leave the ground together