I always found it surprising, a QH farm I used to show for as a teenager, the owner let the horses tails be a cockleburr nest most the year. Now I have a serious cockleburr aversion and I know I over-comb/brush my horses manes and tails, but I don't show so no biggy. But she would just leave the cockleburrs and swear they protected the horses tails. I don't know how valid that is, but her horses tails, once combed out, were always amazingly long and gorgeous!
A diet with vitamins that stimulate hair growth can help. Or a shampoo that has the vitamins. Sometimes diet can effect hair growth. I know since I switched my mare to a different feed she's gotten a really nice mane and tail, and her coat shines so nicely. Also, trim the ends every couple months, condition it and all that. DON'T brush. For Saddlebreds, we pick out their tail, and I do that in hunters. It makes their tail 100 times more full and it doesnt break the tail.
I was going to suggest braiding and wrapping as well, but considering she only has a few inches of hair below the tail bone, that may prove problematic haha. Ideally you should never braid directly on the tailbone (it should start roughly 2 inches below the tailbone) as it will pull out the hairs and can be uncomfortable. And because it's summer she will be swishing her tail a lot. You could try though. Just apply lottttsss of flyspray.
Wash it, let it dry, apply MTG, braid and wrap.
Something you can do to help is wash it and condition it weekly REALLY well and make sure that you rinse it completely out. Then braid it and bag it up. Supplement wise I use Farriers Formula. It is cheap and the same ingredients that help healthy hoof growth also help healthy hair growth.
These aren't the best examples but they are the best that I have. My old horse Pilot, when I originally bought him his hair was stained yellow and was stingy brittle and in horrible condition. These are the before and afters. His mane grew about an extra 6 inches and became in much better condition and his tail grew about 3 feet all in one year. I didn't use any hot oil treatments or any MTG (cause makes the hair folicles dirty and gunky which doesn't promote healthy hair growth). I just washed it very well diligently every week. Only fingered out knots and NEVER used a mane comb or brush (because that damages and breaks hairs) Never used rubberbands for same reason always when braided or banded up used yarn or string. And good feed and Farriers formula supplement.
First day I bought him (I know can't see the tail very well but you can see how damaged and yellow it was, and how stringy and damaged his mane was):
This is right before I had to sell him after 1 year (healthy, white, long, and full) he hadn't actually had a bath for 2 weeks when these were taken because I was out of town:
Tail (he always carried it high but when standing it dragged the ground by a foot):