Is there any scabbing of the tailbone? My mare, out of the blue, ended up with scabbing all over her tailbone this winter. I still don't know the cause, as her tail was clean and my other horse was fine, but I applied Equus microtek twice a day for a few weeks and the itching and scabbing stopped. I have used MTG on my thin skinned arabs with no problem, but I'm not really sure if it worked or not....maybe a little, but it does attract the dirt because it is greasy-- and makes your horse smell like bacon!
Unfortunately, time is really the only way that I've found to get the hairs to grow back. Keeping it clean and making sure it isn't dirty and itchy is really the best method to let the tail grow out again.