pacer (?), n. One who, or that which, paces.[1913 Webster]
2. Especially: A horse trained to a special gait in which both feet on one side leave the ground together; a horse that paces. [wns=2][WordNet 1.5]
3. A horse used to set the pace in racing. [wns=3]Syn. -- pacemaker, pacesetter.[WordNet 1.5]