The Collaborative International Dictionary of English v.0.48:

Mare \Mare\, n. [AS. mara incubus; akin to OHG. & Icel. mara;
   cf. Pol. mora, Bohem. m[*u]ra.] (Med.)
   Sighing, suffocative panting, intercepted utterance, with a
   sense of pressure across the chest, occurring during sleep;
   the incubus; -- obsolete, except in the compound nightmare.
   [1913 Webster]

         I will ride thee o' nights like the mare. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mare \Mare\ (m[^a]r), n. [OE. mere, AS. mere, myre, fem of AS.
   mearh horse, akin to D. merrie mare, G. m[aum]hre, OHG. marah
   horse, meriha mare, Icel. marr horse, OCelt. marka (Pausan.
   19, 19,4), Ir. marc, W. march. Cf. Marshal.]
   The female of the horse and other equine quadrupeds.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

maria \maria\ n. pl.
   1. (Astron.) Plural form of mare, a dark region of
      considerable extent on the surface of the moon.
      [WordNet 1.5]

   2. (Bot.) A valuable timber tree of Panama.

   Syn: Calophyllum longifolium.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mare
    n 1: female equine animal [syn: mare, female horse]
    2: a dark region of considerable extent on the surface of the
       moon [syn: mare, maria]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "mare":
   Houyhnhnm, Partlet, alkali flat, alluvial plain, basin, biddy,
   bitch, bossy, bottomland, brood mare, bushveld, campo, champaign,
   champaign country, charger, coastal plain, colt, courser, cow,
   critter, delta, desert, dobbin, doe, down, downs, entire,
   entire horse, equine, ewe, ewe lamb, fell, filly, flat,
   flat country, flatland, flats, foal, gelding, grass veld,
   grassland, guinea hen, gyp, heath, heifer, hen, hind, horse, jenny,
   lande, level, lioness, llano, lowland, lowlands, lunar mare, mesa,
   mesilla, moor, moorland, mount, nag, nanny, nanny goat,
   open country, pampa, pampas, peahen, peneplain, plain, plains,
   plateau, playa, prairie, prancer, roe, salt flat, salt marsh,
   salt pan, savanna, sebkha, she-bear, she-goat, she-lion, slut, sow,
   stallion, steed, steppe, stud, studhorse, table, tableland, tarpan,
   tigress, top horse, tree veld, tundra, upland, vega, veld, vixen,
   war-horse, weald, wide-open spaces, wild horse, wold