The Collaborative International Dictionary of English v.0.48:

pacer \pa"cer\, 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. [WordNet sense 2]
      [WordNet 1.5]

   3. A horse used to set the pace in racing. [WordNet sense 3]

   Syn: pacemaker, pacesetter.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

PACER
       Public Access to Court Electronic Records