Neatorama has a small gallery of horse mustaches.
One has to assume that at one time there was an evolutionary advantage to having a mustache*. Question for those of you who have horses - do you clip/shave these? or are they best left alone?
* I was trying to work out an etymology from "mus" = mouse + "tache" = pocket. Didn't work.
1580s, from French moustache (15c.), from Italian mostaccio, from Medieval Greek moustakion, diminutive of Doric mystax (genitive mystakos) "upper lip, mustache," related to mastax "jaws, mouth," literally "that with which one chews," from PIE root *mendh- "to chew" (see mandible). Borrowed earlier (1550s) as mostacchi, from the Italian word or its Spanish derivative mostacho. The plural form of this, mustachios, lingers in English. Slang shortening stache attested from 1985.