Mix hot sauce and vasoline and rub on mares tail. I had some young ones do this to my guys tail. 1st I tried tail bags and it just got pulled off along with lots of tail. This was the only thing that worked and it worked quick.
Hope it helps.
I'm not sure what to do for a chewed tail, but sounds to me like the hot sauce/vaseline would deter the yearling! One thing that I would check though, is to make sure your girlie is getting the nutrition that she needs. Sometimes they will chew wood or tails because they aren't getting the nutrients that they need, but, of course, sometimes they will do it just out of boredom.