The Collaborative International Dictionary of English v.0.48:

Appellor \Ap`pel*lor"\ ([a^]p`p[e^]l*l[^o]r"), n. [OF. apeleur,
   fr. L. appellator, fr. appellare.] (Law)
   (a) The person who institutes an appeal, or prosecutes
       another for a crime. --Blackstone.
   (b) One who confesses a felony committed and accuses his
       accomplices. --Blount. --Burrill.
       [1913 Webster]

   Note: This word is rarely or never used for the plaintiff in
         appeal from a lower court, who is called the
         appellant. Appellee is opposed both to appellant
         and appellor.
         [1913 Webster]

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

APPELLOR. A criminal who accuses his accomplices; one who challenges a jury.