Oh god! Bob is not nearly that bad. He only does it inside or outside when a) all his friends are gone or b) all the food is gone. That one time was the only time Ive seen him do it on metal, and we were stabled at a show for 3 days straight. There was nowhere else to do it, but he hated the metal, he would latch on and then get even more mad because it probably hurt his teeth.

A mare at my barn will do it on her stall guard.