The Collaborative International Dictionary of English v.0.48:

Libelant \Li"bel*ant\ (-ant), n.
   One who libels; one who institutes a suit in an
   ecclesiastical or admiralty court. [Written also
   libellant.] --Cranch.
   [1913 Webster]

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

LIBELLANT. The party who fires a libel in a chancery or admiralty case,
corresponds to the plaintiff in actions in the common law courts, is called
the libellant.