noun (n)

• a horse used to set the pace in racing(noun.animal)

  Synonym:
  pacemaker, pacesetter
  

  source: wordnet30
• a horse trained to a special gait in which both feet on one side leave the ground together(noun.animal)

  source: wordnet30
• One who, or that which, paces; especially, a horse that paces.(noun)

  source: webster1913