The Collaborative International Dictionary of English v.0.48:

Horse \Horse\ (h[^o]rs), n. [AS. hors; akin to OS. hros, D. &
   OHG. ros, G. ross, Icel. hross; and perh. to L. currere to
   run, E. course, current Cf. Walrus.]
   1. (Zool.) A hoofed quadruped of the genus Equus;
      especially, the domestic horse (Equus caballus), which
      was domesticated in Egypt and Asia at a very early period.
      It has six broad molars, on each side of each jaw, with
      six incisors, and two canine teeth, both above and below.
      The mares usually have the canine teeth rudimentary or
      wanting. The horse differs from the true asses, in having
      a long, flowing mane, and the tail bushy to the base.
      Unlike the asses it has callosities, or chestnuts, on all
      its legs. The horse excels in strength, speed, docility,
      courage, and nobleness of character, and is used for
      drawing, carrying, bearing a rider, and like purposes.
      [1913 Webster]

   Note: Many varieties, differing in form, size, color, gait,
         speed, etc., are known, but all are believed to have
         been derived from the same original species. It is
         supposed to have been a native of the plains of Central
         Asia, but the wild species from which it was derived is
         not certainly known. The feral horses of America are
         domestic horses that have run wild; and it is probably
         true that most of those of Asia have a similar origin.
         Some of the true wild Asiatic horses do, however,
         approach the domestic horse in several characteristics.
         Several species of fossil (Equus) are known from the
         later Tertiary formations of Europe and America. The
         fossil species of other genera of the family
         Equid[ae] are also often called horses, in general
         sense.
         [1913 Webster]

   2. The male of the genus Equus, in distinction from the
      female or male; usually, a castrated male.
      [1913 Webster]

   3. Mounted soldiery; cavalry; -- used without the plural
      termination; as, a regiment of horse; -- distinguished
      from foot.
      [1913 Webster]

            The armies were appointed, consisting of twenty-five
            thousand horse and foot.              --Bacon.
      [1913 Webster]

   4. A frame with legs, used to support something; as, a
      clotheshorse, a sawhorse, etc.
      [1913 Webster]

   5. A frame of timber, shaped like a horse, on which soldiers
      were made to ride for punishment.
      [1913 Webster]

   6. Anything, actual or figurative, on which one rides as on a
      horse; a hobby.
      [1913 Webster]

   7. (Mining) A mass of earthy matter, or rock of the same
      character as the wall rock, occurring in the course of a
      vein, as of coal or ore; hence, to take horse -- said of a
      vein -- is to divide into branches for a distance.
      [1913 Webster]

   8. (Naut.)
      (a) See Footrope, a.
      (b) A breastband for a leadsman.
      (c) An iron bar for a sheet traveler to slide upon.
      (d) A jackstay. --W. C. Russell. --Totten.
          [1913 Webster]

   9. (Student Slang)
      (a) A translation or other illegitimate aid in study or
          examination; -- called also trot, pony, Dobbin.
      (b) Horseplay; tomfoolery.
          [Webster 1913 Suppl.]

   10. heroin. [slang]
       [PJC]

   11. horsepower. [Colloq. contraction]
       [PJC]

   Note: Horse is much used adjectively and in composition to
         signify of, or having to do with, a horse or horses,
         like a horse, etc.; as, horse collar, horse dealer or
         horse?dealer, horsehoe, horse jockey; and hence, often
         in the sense of strong, loud, coarse, etc.; as,
         horselaugh, horse nettle or horse-nettle, horseplay,
         horse ant, etc.
         [1913 Webster]

   Black horse, Blood horse, etc. See under Black, etc.

   Horse aloes, caballine aloes.

   Horse ant (Zool.), a large ant (Formica rufa); -- called
      also horse emmet.

   Horse artillery, that portion of the artillery in which the
      cannoneers are mounted, and which usually serves with the
      cavalry; flying artillery.

   Horse balm (Bot.), a strong-scented labiate plant
      (Collinsonia Canadensis), having large leaves and
      yellowish flowers.

   Horse bean (Bot.), a variety of the English or Windsor bean
      (Faba vulgaris), grown for feeding horses.

   Horse boat, a boat for conveying horses and cattle, or a
      boat propelled by horses.

   Horse bot. (Zool.) See Botfly, and Bots.

   Horse box, a railroad car for transporting valuable horses,
      as hunters. [Eng.]

   Horse breaker or Horse trainer, one employed in subduing
      or training horses for use.

   Horse car.
       (a) A railroad car drawn by horses. See under Car.
       (b) A car fitted for transporting horses.

   Horse cassia (Bot.), a leguminous plant (Cassia
      Javanica), bearing long pods, which contain a black,
      catharic pulp, much used in the East Indies as a horse
      medicine.

   Horse cloth, a cloth to cover a horse.

   Horse conch (Zool.), a large, spiral, marine shell of the
      genus Triton. See Triton.

   Horse courser.
       (a) One that runs horses, or keeps horses for racing.
           --Johnson.
       (b) A dealer in horses. [Obs.] --Wiseman.

   Horse crab (Zool.), the Limulus; -- called also
      horsefoot, horsehoe crab, and king crab.

   Horse crevall['e] (Zool.), the cavally.

   Horse emmet (Zool.), the horse ant.

   Horse finch (Zool.), the chaffinch. [Prov. Eng.]

   Horse gentian (Bot.), fever root.

   Horse iron (Naut.), a large calking iron.

   Horse latitudes, a space in the North Atlantic famous for
      calms and baffling winds, being between the westerly winds
      of higher latitudes and the trade winds. --Ham. Nav.
      Encyc.

   Horse mackrel. (Zool.)
       (a) The common tunny (Orcynus thunnus), found on the
           Atlantic coast of Europe and America, and in the
           Mediterranean.
       (b) The bluefish (Pomatomus saltatrix).
       (c) The scad.
       (d) The name is locally applied to various other fishes,
           as the California hake, the black candlefish, the
           jurel, the bluefish, etc.

   Horse marine (Naut.), an awkward, lubbery person; one of a
      mythical body of marine cavalry. [Slang]

   Horse mussel (Zool.), a large, marine mussel (Modiola
      modiolus), found on the northern shores of Europe and
      America.

   Horse nettle (Bot.), a coarse, prickly, American herb, the
      Solanum Carolinense.

   Horse parsley. (Bot.) See Alexanders.

   Horse purslain (Bot.), a coarse fleshy weed of tropical
      America (Trianthema monogymnum).

   Horse race, a race by horses; a match of horses in running
      or trotting.

   Horse racing, the practice of racing with horses.

   Horse railroad, a railroad on which the cars are drawn by
      horses; -- in England, and sometimes in the United States,
      called a tramway.

   Horse run (Civil Engin.), a device for drawing loaded
      wheelbarrows up an inclined plane by horse power.

   Horse sense, strong common sense. [Colloq. U.S.]

   Horse soldier, a cavalryman.

   Horse sponge (Zool.), a large, coarse, commercial sponge
      (Spongia equina).

   Horse stinger (Zool.), a large dragon fly. [Prov. Eng.]

   Horse sugar (Bot.), a shrub of the southern part of the
      United States (Symplocos tinctoria), whose leaves are
      sweet, and good for fodder.

   Horse tick (Zool.), a winged, dipterous insect (Hippobosca
      equina), which troubles horses by biting them, and
      sucking their blood; -- called also horsefly, horse
      louse, and forest fly.

   Horse vetch (Bot.), a plant of the genus Hippocrepis
      (Hippocrepis comosa), cultivated for the beauty of its
      flowers; -- called also horsehoe vetch, from the
      peculiar shape of its pods.

   Iron horse, a locomotive. [Colloq.]

   Salt horse, the sailor's name for salt beef.

   To look a gift horse in the mouth, to examine the mouth of
      a horse which has been received as a gift, in order to
      ascertain his age; -- hence, to accept favors in a
      critical and thankless spirit. --Lowell.

   To take horse.
       (a) To set out on horseback. --Macaulay.
       (b) To be covered, as a mare.
       (c) See definition 7 (above).
           [1913 Webster]

WordNet (r) 3.0 (2006):

Equus caballus
    n 1: solid-hoofed herbivorous quadruped domesticated since
         prehistoric times [syn: horse, Equus caballus]