Definify.com
Webster 1913 Edition
Mare
Mare
(mâr)
, Noun.
 [OE. 
mere
, AS. mere
, myre
, fem of AS. mearh 
horse, akin to D. merrie 
mare, G. mähre
, 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. 
 Mare
,Noun.
 [AS. 
mara 
incubus; akin to OHG. & Icel. mara
; cf. Pol. mora
, Bohem. můra
.] (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
. Shakespeare
Webster 1828 Edition
Mare
MARE
,Noun.
  A kind or torpor or stagnation which seems to press the stomach in sleep; the incubus. [It is now used only in the compound, nightmare, which ought to be written nightmar.]