c.1290, " an evil female spirit afflicting sleepers with a feeling of suffocation," compounded from night + mare " goblin that causes nightmares, incubus," from O.E. mare " incubus," from mera, mære, from P.Gmc. *maron " goblin," from PIE *mora- " incubus," from base *mer- " to rub away, harm, seize" ( cf. first element in O.Ir. Morrigain " demoness of the corpses," lit. " queen of the nightmare," also Bulg., Serb., Pol. mora " incubus;" Fr. cauchemar, with first element is from O.Fr. caucher " to trample"). Meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of " any bad dream" first recorded 1829; that of " very distressing experience" is from 1831.
Voila, le mare ne vient pas du tout de " jument", mais de"mer" " mora" comme expliqué ci-dessus.
C´est pareil en français caucheMAR