D defs.my
Entry 3 senses Webster, 1913

pacer

/pās'-ẽr/ · pa·cer · IPA /ˈpeɪsəɹ/
01 n. One who, or that which, paces.
  1. 1.
    One who, or that which, paces.
  2. 2.
    A horse trained to a special gait in which both feet on one side leave the ground together; a horse that paces.
  3. 3.
    A horse used to set the pace in racing.