Ferocious, a.
Etymology: [L. ferox, -ocis, fierce: cf. F. féroce. See Ferocity.]
Fierce; savage; wild; indicating cruelty; ravenous; rapacious; as, ferocious look or features; a ferocious lion. The humbled power of a ferocious enemy. Lowth.
Syn. -- Ferocious, Fierce, Savage, Barbarous. 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. It [Christianity] has adapted the ferociousness of war. Blair.
Shades of meaning
Fernald has no entry of its own for ferocious; he lists it under fierce.
Where he writes about it:
Ferocious refers to a state or disposition; that which is fierce flashes or blazes; that which is ferocious steadily burns; we speak of a ferocious animal, a fierce passion. — from fierce
A fiery spirit with a good disposition is quickly excitable in a good cause, but may not be fierce or ferocious. — from fierce
Ferocious always denotes a tendency to violence; it is more distinctly bloodthirsty than the other words; a person may be deeply, intensely cruel, and not at all ferocious; a ferocious countenance expresses habitual ferocity; a fierce countenance may express habitual fierceness, or only the sudden anger of the moment. — from fierce