The Collaborative International Dictionary of English v.0.48:

nightrider \nightrider\ n.
   member of a secret mounted band in the southern U. S. after
   the Civil War, which committed acts of intimidation and
   revenge.

   Syn: night rider.
        [WordNet 1.5] nightrobe

WordNet (r) 3.0 (2006):

nightrider
    n 1: member of a secret mounted band in United States South
         after the American Civil War; committed acts of
         intimidation and revenge [syn: night rider, nightrider]