Horse fly

A horse fly is a type of insect and a member of the family Tabanidae (order Diptera). More specifically they are any member of the genus "Tabanus". These stout flies are as small as a housefly or as large as a bumblebee. Their metallic or iridescent eyes meet dorsally in the male and are separate in the female. Gad-fly, a nickname, may refer either to the fly’s roving habits or to its mouthparts, which resemble a wedge-shaped miner’s tool. 
The bites of the female fly often carry parasites onto the victim (such as a horse). The resulting sore can be extremely distressing to the animal.