When I first got my horse, her tail looked a bit like this. I really like Frutis Sleek & Shine leave-in conditioner. My horse's mane is much thicker and longer, and her tail has stopped breaking off since I've started using the stuff regularly. Every time I get to go to the barn (which is about twice a week), I put a bunch of it on her mane, forlock, and usually her tail--right on to the dry hair. Then I comb it with a mane comb which has teeth that spin, so as not to pull hair out--it just combs right through.
I had used Head and Shoulders the first few shampoos (very diluted in a bucket of water) because it really gets the dandruff and build-up off the dock better than regular horsey shampoos. You have to rinse it really good and get all the soap out or else the horse will just get itchy there and scratch his butt, then the tail hair breaks, etc. lol. Fructis leave-in conditioner can be used on wet or dry hair. I never have to wrap, braid, or bag Brandy's tail because it is growing well without those since I've started using Fructis Sleek & Shine.
**I must not forget to thank the difficult horses, who made my life miserable, but who were better teachers than the well-behaved school horses who raised no problems.**