Add-on- I typically brush manes and tails with a round-headed brush, not a human one. My absolute favorite is the Conair Equine Pro. Oster's brushes come in second. I'm actually not a huge fan of combs. Ime they tend to rip out knots instead of actually detangling them.
I'm going to go against the grain here and say that I will brush manes and tails, but always starting at the bottom and working my way up after I untangle the worst snarls with my fingers. Avoid any sprays and detanglers with silicone, dimethicone, or really anything that ends in -one unless you have to deal with really bad tangles. It coats the hair shaft and can cause it to become brittle and break. Bagging up the tail in winter can also help protect it, and of course, nice manes and tails always start with good nutrition!
To grow your horses tail, don’t brush it every day. Even though it looks nice, it breaks the hair and thins the tail. Maybe brush it once or twice a week, and with a wide-toothed comb. What you should do though is brush the dock and upper tail bone with a dandy brush daily. It will gently get rid of any dandruff or dirt that may feel uncomfortable for them. They may start to itch/rub their tail if it gets very dirty, which will break a lot of hair in their tail. Brushing with the dandy brush also stimulates blood flow, which creates growth.
I hope this helps answer your question!
I prefer to not bush the mane and tail and if you do thoroughly spary the hair with detangler first, let it dry then gently brush it out.
To growing a long and hick tail I prefer to use a comb instead of a brush. I also like to wash the tail or mane, I let the conditioner sit in the mane or tail then comb then rinse then comb again. One thing that I really think helps is to keep the mane and tail trimmed.