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]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "guarantor":
   angel, approver, backer, bailsman, bondsman, certifier, confirmer,
   cosignatory, cosigner, endorser, godfather, godparent, guarantee,
   guaranty, insurer, mainpernor, mortgagor, notary, notary public,
   party, patron, ratifier, signatory, sponsor, subscriber, surety,
   underwriter, upholder, warrantor



Bouvier's Law Dictionary, Revised 6th Ed (1856):

GUARANTOR, contracts. He who makes a guaranty.
     2. The guarantor is bound to fulfill the engagement he has entered into,
provided the principal debtor does not. He is bound only to the extent that
the debtor is, and any payment made by the latter, or release of him by the
creditor, will operate as a release of the guarantor; 3 Penna. R. 19; or
even if the guarantee should give time to the debtor beyond that contained
in the agreement, or substitute a new agreement, or do any other act by
which the guarantor's situation would be worse, the obligation of the latter
would be discharged. Smith on Mer. Law, 285.
     3. A guarantor differs from a surety in this, that the former cannot be
sued until a failure on the part of the principal, when sued; while the
latter may be sued at the same time with the principal. 10 Watts, 258.