mare (n.1) Look up mare at Dictionary.com
"female horse," Old English mere (Mercian), myre (West Saxon), fem. of mearh "horse," from Proto-Germanic *markhjon- (cognates: Old Saxon meriha, Old Norse merr, Old Frisian merrie, Dutch merrie, Old High German meriha, German Mähre "mare"), said to be of Gaulish origin (compare Irish and Gaelic marc, Welsh march, Breton marh "horse"). No known cognates beyond Germanic and Celtic. As the name of a throw in wrestling, it is attested from c.1600. Mare's nest "illusory discovery, excitement over something which does not exist" is from 1610s.
mare (n.2) Look up mare at Dictionary.com
"broad, dark areas of the moon," 1765, from Latin mare "sea" (see marine), applied to lunar features by Galileo and used thus in 17c. Latin works. They originally were thought to be actual seas.
mare (n.3) Look up mare at Dictionary.com
"night-goblin, incubus," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" (cognates: Middle Low German mar, Middle Dutch mare, Old High German mara, German Mahr "incubus," Old Norse mara "nightmare, incubus"), from PIE *mora- "incubus" (cognates: first element in Old Irish Morrigain "demoness of the corpses," literally "queen of the nightmare," also Bulgarian, Serbian mora, Czech mura, Polish zmora "incubus;" French cauchemar, with first element from Old French caucher "to trample"), from root *mer- "to rub away, harm" (see morbid).