The Collaborative International Dictionary of English v.0.48:

Ferocious \Fe*ro"cious\, a. [L. ferox, -ocis, fierce: cf. F.
   f['e]roce. See Ferocity.]
   Fierce; savage; wild; indicating cruelty; ravenous;
   rapacious; as, ferocious look or features; a ferocious lion.
   [1913 Webster]

         The humbled power of a ferocious enemy.  --Lowth.

   Syn: Ferocious, Fierce, Savage, Barbarous.

   Usage: When these words are applied to human feelings or
          conduct, ferocious describes the disposition; fierce,
          the haste and violence of an act; barbarous, the
          coarseness and brutality by which it was marked;
          savage, the cruel and unfeeling spirit which it
          showed. A man is ferocious in his temper, fierce in
          his actions, barbarous in the manner of carrying out
          his purposes, savage in the spirit and feelings
          expressed in his words or deeds. -- Fe*ro"cious*ly,
          adv. -- Fe*ro"cious*ness, n.
          [1913 Webster]

WordNet (r) 3.0 (2006):

ferocious
    adj 1: marked by extreme and violent energy; "a ferocious
           beating"; "fierce fighting"; "a furious battle" [syn:
           ferocious, fierce, furious, savage]

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "ferocious":
   Draconian, Tartarean, abandoned, aggressive, amok, animal,
   antagonistic, anthropophagous, atrocious, barbaric, barbarous,
   battling, beastly, bellicose, belligerent, bellowing, berserk,
   bestial, bloodthirsty, bloody, bloody-minded, brutal, brutalized,
   brute, brutish, cannibalistic, carried away, chauvinist,
   chauvinistic, combative, contentious, cruel, cruel-hearted,
   delirious, demoniac, demoniacal, destructive, devilish, diabolic,
   diabolical, distracted, ecstatic, enemy, enraptured, fell, feral,
   ferine, fiendish, fiendlike, fierce, fighting, frantic, frenzied,
   full of fight, fulminating, furious, grim, haggard, hawkish,
   hellish, hog-wild, hostile, howling, hysterical, implacable,
   in a transport, in hysterics, infernal, inhuman, inhumane,
   inimical, intoxicated, jingo, jingoish, jingoist, jingoistic,
   kill-crazy, mad, madding, malign, malignant, maniac, martial,
   merciless, militant, militaristic, military, monstrous, murderous,
   noncivilized, offensive, orgasmic, orgiastic, pitiless, possessed,
   predatory, pugnacious, quarrelsome, rabid, raging, ramping,
   ranting, rapacious, ravening, ravenous, raving, ravished,
   relentless, roaring, running mad, ruthless, saber-rattling,
   sadistic, sanguinary, sanguineous, satanic, savage, scrappy,
   sharkish, slavering, soldierlike, soldierly, storming, subhuman,
   tameless, transported, trigger-happy, truculent, unchristian,
   uncivilized, uncontrollable, unfriendly, ungentle, unhuman,
   unpacific, unpeaceable, unpeaceful, untamed, vicious, violent,
   voracious, warlike, warmongering, warring, wild, wild-eyed,
   wild-looking, wolfish