I've heard Bio Oil is great for scars :)
I would try vitamin E oil from grocery store vitamin section. Does wonders on people scars and is food safe so I imagine would be okay for horse.
I use MTG on my horse and it works well! But not all scars grow back hair.
You could try neem oil or stinky stuff as they promote hair growth :) some scars don't heal, I had a gypsy cob who had been hobbled so had a scar on one of his hind legs, the hair never grew back and we put Vaseline on it when the skin got dry