The Collaborative International Dictionary of English v.0.48:

Mettlesome \Met"tle*some\, a.
   Full of spirit; possessing constitutional ardor; fiery; as, a
   mettlesome horse.
   [1913 Webster] -- Met"tle*some*ly, adv. --
   Met"tle*some*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mettlesome
    adj 1: having a proud and unbroken spirit
    2: willing to face danger [syn: game, gamy, gamey,
       gritty, mettlesome, spirited, spunky]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "mettlesome":
   active, acute, aggressive, agitable, animated, beany, brisk,
   bulldoggish, courageous, dauntless, dynamic, edgy, emotional,
   emotionally unstable, energetic, enterprising, enthusiastic,
   eruptive, excitable, explosive, fiery, forceful, forcible,
   full of pep, game, gamy, go-go, gritty, gutsy, gutty, hearty,
   high-mettled, high-spirited, high-strung, highly emotional,
   impetuous, incisive, inflammable, intense, irascible, irritable,
   keen, kinetic, lively, living, lusty, nervous, nervy, peppery,
   peppy, pertinacious, perturbable, plucky, prickly, red-blooded,
   resolute, robust, sensitive, skittish, smacking, snappy, spanking,
   spirited, spiritful, spunky, startlish, strenuous, strong,
   take-charge, take-over, tenacious, touchy, tough, trenchant,
   vibrant, vigorous, vivacious, vivid, volcanic, zestful, zesty,
   zippy