The Collaborative International Dictionary of English v.0.48:

Steed \Steed\ (st[=e]d), n. [OE. stede, AS. st[=e]da a
   stud-horse, war horse, fr. st[=o]d a stud of breeding steeds;
   akin to G. stute a mare, Icel. stedda, st[=o][eth], a stud.
   [root]163. See Stud of horses.]
   A horse, especially a spirited horse for state or war; --
   used chiefly in poetry or stately prose. "A knight upon a
   steed." --Chaucer.
   [1913 Webster]

         Mounted upon a hot and fiery steed.      --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

steed
    n 1: (literary) a spirited horse for state or war

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "steed":
   Houyhnhnm, brood mare, charger, colt, courser, critter, dobbin,
   entire, entire horse, equine, filly, foal, gelding, horse, mare,
   mount, nag, prancer, stallion, stud, studhorse, tarpan, top horse,
   war-horse, wild horse