The Collaborative International Dictionary of English v.0.48:

Guarantor \Guar"an*tor`\, n. [See Guaranty, and cf.
   Warrantor.] (Law)
      (a) One who makes or gives a guaranty; a warrantor; a
          surety.
      (b) One who engages to secure another in any right or
          possession.
          [1913 Webster]

WordNet (r) 3.0 (2006):

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