I have had a lot of luck with using Shapley’s Original M-T-G for growing hair back on horses and dogs. I’ve used it on almost everything (mane, tail, girth itch, ect) The hair grows back thick and there are no signs the hair was ever missing. I like to comb it in with a wide toothed comb to avoid loosing any more hair.
The one thing I would mention is it is greasy so be prepared to wear gloves when applying. It washes off fine with soap but being oily everything sticks to you on the way to the sink.
I hope this helps