Even when my horse was in a paddock by himself he still managed to get it off. When he was out with other horses I stopped bothering with it, because I would have to search for it on the ground shortly afterwards.
Day 1: I had to hunt the mask down in the pasture. Day 2: Ears sticking out of hole intended for forelock. I haven't used it since. Because the ears are made of soft material, they have no form and slip easily off the horse's ears which defeats the purpose of "w/ears."