i had one buzzing around my horse years ago at a show. she was terrified of it. i got a really good look at it because it flew quite slowly, and emailed the "bug dept" of the National History Museum because i thought it must be an illegal immegrant! it made a really loud droning noise as it flew, honest to god it was huge and there's no way you could mistake it for a normal one.
eugh.
Its the Gad Fly or Deer Fly and belongs to the Genus or family of Tabanidae the blood feeding flies.
We get them here in the Highlands and they are enormous about 1.5 cm long.
I found one sitting on my horses hip one day , having sliced into the flesh with its mandibles and it was lapping up the dripping blood!
Horrendous nightmares. I swatted it then stamped on it and was still able to do an identification from the crumpled remains.
My horse goes mental if one approaches him, they make a hell of a buzz, and I don't blame him one bit.