The Collaborative International Dictionary of English v.0.48:

Thoroughbred \Thor"ough*bred`\, a.
   Bred from the best blood through a long line; pure-blooded;
   -- said of stock, as horses. Hence, having the
   characteristics of such breeding; mettlesome; courageous; of
   elegant form, or the like. -- n. A thoroughbred animal,
   especially a horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thoroughbred
    adj 1: having a list of ancestors as proof of being a purebred
           animal [syn: pedigree(a), pedigreed, pureblood,
           pureblooded, thoroughbred]
    n 1: a well-bred person
    2: a racehorse belonging to a breed that originated from a cross
       between Arabian stallions and English mares
    3: a pedigreed animal of unmixed lineage; used especially of
       horses [syn: thoroughbred, purebred, pureblood]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "thoroughbred":
   American trotter, Appaloosa, Arabian, Barb, Barbary horse, Belgian,
   Brahman, Cleveland Bay, French coach horse, Galloway, Hambletonian,
   Shetland, Shetland pony, Shire, Suffolk, Turk, archduke,
   aristocrat, armiger, baron, baronet, blue blood, blue-blooded,
   count, daimio, duke, earl, esquire, full-blooded, genet, gentleman,
   grand duke, grandee, hidalgo, highborn, highbred, jennet,
   lace-curtain, laird, landgrave, lord, lordling, magnate, magnifico,
   margrave, marquis, noble, nobleman, of good breed, optimate,
   palomino, palsgrave, patrician, pedigree, pedigreed, peer,
   pur sang, pure-blooded, purebred, quarter horse, saddle horse,
   seigneur, seignior, silk-stocking, squire, swell, upper-cruster,
   viscount, waldgrave, well-bred, wellborn