Horse flies (Tabanidae) are also called breeze flies, clegs, klegs, or clags, deer- and gad fly.
Horse flies are strong, burly flies with big eyes. The larvae develop in water, mud, swamp or decaying plant parts. They often live in other insects.
They settle down quickly and then bite almost immediately, and painfull, perceptable. They may bite through thin clothing around. Horse flies are fast (25 km / h) and chase their prey persistent, sometimes a mile long. After a bit, they are slow and you can easily kill them.

People who do not hurt a fly just are not fast enough.