The Collaborative International Dictionary of English v.0.48:

Covenantor \Cov"e*nant*or`\ (-?r`), n. (Law)
   The party who makes a covenant. --Burrill.
   [1913 Webster]

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

COVENANTOR. One who becomes bound to perform a covenant.
     2. To become a covenantor a person must be sui juris, and intend, at
the time of becoming bound, to covenant to perform some act mentioned in the
covenant. He can be discharged from his covenant by performance, or, by the
act of the covenantee, as the non-performance of a condition precedent, a
release, or a rescission of the contract.