horsefly

/ˈhɔːsˌflaɪ/
noun (pl) -flies
1.
any large stout-bodied dipterous fly of the family Tabanidae, the females of which suck the blood of mammals, esp horses, cattle, and man Also called gadfly, cleg
Word Origin and History for horsefly
n.

late 14c., from horse (n.) + fly (n.).