I was hoping to find an answer here how to manage to keep a horse from removing not only his, but his pasturemate's flymask and making them disappear.
There are those criminal element herd mates that are very clever at removing someone else's fly mask.
I prefer Farnum Super Mask because they have double closures and I tighten them "three fingers tight" between the eyeball and the ear.
I have tried Cashel with the long nose and also with ears. My horse that needed that mask never tried to get his Farnum Mask off but, everyday around mid-afternoon, I would watch him roll and he would rub his head along the ground until he managed to get the Cashel off
He wanted nothing to do with the extended nose piece, nor the ear covers. Soon as I put his plain old Farnum back on him, he left the mask alone.
If he starts rubbing his head, it's because he has an itch somewhere and that itch is generally a tick
If your horse's ears are also white (some Paints and Spotted's have a Medicine Hat), you might want to buy the ears separate from the face mask and see if that at least helps keep the face mask on.
Also buy a brand of face mask that has the double closure, so his herd mate can't pull it off