If this is bigger, it is bigger than ordinary flies. That's Niu Meng! If it is very small, it may be a fruit fly. Because I don't know yours. How big is this? You can't tell the difference between these two insects, yourself. For reference.

If this kind of cow protects itself, it is very powerful to suck blood. Even on the skin of a cow, it can bite the skin and suck his blood.