horseÂ·fly. also horse fly (hÃ´rsâ²flÄ«â²) n. Any of numerous large biting tabanid flies, the females of which feed on the blood of various mammals. horsefly. (ËhÉËsËflaÉª) n, pl -flies. (Animals) any large stout-bodied dipterous fly of the family Tabanidae, the females of which suck the blood of mammals, esp horses, cattle, and man.