The Collaborative International Dictionary of English v.0.48:

Warranter \War"rant*er\, n.
   1. One who warrants, gives authority, or legally empowers.
      [1913 Webster]

   2. (Law) One who assures, or covenants to assure; one who
      contracts to secure another in a right, or to make good
      any defect of title or quality; one who gives a warranty;
      a guarantor; as, the warranter of a horse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

warranter
    n 1: one who provides a warrant or guarantee to another [syn:
         guarantor, surety, warrantor, warranter]