My horse's have had fly masks at the barn where I boarded, but I had tried some on them, so they were used to them. But since they moved home the flies have been really bad on their faces (I blame the cows next door).
Anyway, I put one on my QH gelding, Beau, he was fine. Then I went and put one on my TB mare, Daisy. I compared fit and decided Beau needs the next size up so I took his off so it stayed clean so I could exchange it.
I'm in the barn and hear the horses start running around... didn't think much of it. But, then there was really loud squealing going on so I looked out the door. Beau was chasing Daisy and her foal all around the field. It took me about 10 minuted to stop them, catch the gelding and put him in a stall. I took off Daisy's fly mask, turned Beau back out and all was fine.
It was the flymask. And I don't know why he doesn't like other horses with them on...