The Collaborative International Dictionary of English v.0.48:

Stallion \Stal"lion\ (st[a^]l"y[u^]n), n. [OE. stalon, OF.
   estalon, F. ['e]talon, fr. OHG. stal a stable. See Stall,
   n.]
   A male horse not castrated; a male horse kept for breeding.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stallion
    n 1: uncastrated adult male horse [syn: stallion, entire]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "stallion":
   Houyhnhnm, billy, billy goat, boar, brood mare, bubbly-jock, buck,
   bull, bullock, chanticleer, charger, cock, cockerel, colt, courser,
   critter, dobbin, dog, drake, entire, entire horse, equine, filly,
   foal, gander, gelding, gobbler, hart, he-goat, horse, mare, mount,
   nag, peacock, prancer, ram, rooster, stag, steed, steer, stot,
   stud, studhorse, tarpan, tom, tom turkey, tomcat, top cow,
   top horse, tup, turkey gobbler, turkey-cock, war-horse, wether,
   wild horse