braid it and keep it in a braid and it should grow
Don't brush the tail as most people said. Instead pick out the grass or whatever's stuck in your horses tail and run your fingers through the tail. Always let your horses tail dry properly and put detangler in the tail and run your fingers through. Also a tail bag could be helpful. Clean the dock with a sponge so the horse doesn't itch and pull hair out. I hope this helped :)
Don't brush your horse's tail every day!
Bang your horse’s tail to give it a neater, thicker appearance. “Banging” refers to trimming the bottom of your horse’s tail. Banging gives the tail a thick, boxy appearance. Trim it when the hair is wet and combed out. Cut only the longest hairs straight across the bottom.
And generously applying coconut oil every now and then is like magical for tail growth.
Along with what Lauren said, MTG works great, too.
SupremeProducts has a mane and tail builder serum that I've heard is pretty good :)
Don't brush the tail- at all. All it does is pull hairs our. As well as that also don't bathe regularly as that washes out natural oils