Mare \Mare\ (m[^a]r), n. [OE. mere, AS. mere, myre, fem of AS. mearh horse, akin to D. merrie mare, G. m["a]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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

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.

I will ride thee o' nights like the mare. --Shak.

Source: Webster's Revised Unabridged Dictionary (1913)