Definition of horse-fly:
(n) :
A hoofed mammal, Equus ferus caballus, often used throughout history for riding and draft work.
(n) :
Any member of the species Equus ferus, including the Przewalski's horse and the extinct Equus ferus ferus.
(n) :
(zoology) Any current or extinct animal of the family Equidae, including zebras and asses.
(n) :
(military, sometimes uncountable) Cavalry soldiers (sometimes capitalized when referring to an official category).
(n) :
A component of certain games.
(n) :
(chess, informal) The chess piece representing a knight, depicted as a horse.