Hey! I’m currently working with the same issue. My mare has a partially white tail that I’m working with getting the urine stains out of.
The first method I’d suggest is using vinegar and baking soda. First I wash the tail with regular horse shampoo, and then throughly rinse it out. After this I create a paste with baking soda and a whitening shampoo and apply that to the tail (I only add it to the bottom of her tail since that’s the area I’m trying to whiten) and then let it sit for 15-20 minutes. Once that is done I’ll rinse the tail with vinegar. After this I rinse again with water and lightly dry the tail with a towel.
The next method you can use if the stains are really stubborn. Like the first method, I first shampoo and rinse the tail. Next I make a paste out of oxiclean and a whitening shampoo and apply this to the tail. I let this sit for 15-20 minutes, and then rinse throughly with water and dry the tail. A word of warning though with this method, I wouldn’t use this on the tailbone/skin since it might cause irritation. I just use this method on the very end of Jessica’s tail and I’m careful that it doesn’t get anywhere else on her body.
No matter what method you use, I’d definitely suggest you keep your horse’s tail up in a tail bag or a sock. After washing and drying your horses tail, braid it. Let the first 3-4 of braids be loose, and then tighten them and braid all the way down to the end and secure with electrical tape (this’ll prevent breakage which could happen with rubber bands). Next I run a small line of conditioner down the braid and rub it in though the tail the best you can (this will help to re-hydrate the hairs since the treatments will dry them out). Try to avoid getting any conditioner on the tailbone as this might cause some irritation. After this I put the tail in the tail bag and tie tail bag BELOW the tailbone. Make sure to not tie it on the tailbone! Depending on the horse, you can leave it in for a week or two before it has to be redone. It may take a few washes before the tail is completely white again, but that should help! Good luck!