It is fairly common for horses to lose hair in the area where they have received a moderate to severe injury. It is also normal for the hair to take a long time to grow back, or for it to fail to grow back altogether. A large part of whether or not hair will eventually grow back over a scar depends on the severity of the injury and how the wound was treated at the time. Proper and comprehensive wound care can greatly increase the likelihood that your horse's hair will grow back over the scar. There are also an assortment of products that claim to be able to help with the growth.
Examine the wound carefully. Look to see if there are signs that hair has begun to grow back and is just growing slowly -- or if the scar appears completely bare. If there is a significant amount of rough scar tissue formed over the area, it will be more difficult to get hair to grow back.
Select a salve or ointment to apply to the scar. There are a variety of different products on the market for this. Some products are mild irritants that abrade the scar with the intention of encouraging the skin and cells to grow more quickly and produce hair in the process, others are creams for keeping skin soft, so that hair may be able to grow back naturally. Vitamin E lotions and basic Corona ointment are common choices to soften and condition the skin. You may also be able to access prescription ointments through your veterinarian.
Apply salve to horse's scars and injuries, as directed by product instructions. For basic lotions and ointments, you can apply daily. Massage the area where the scar is located to help encourage good blood flow to tissues. Wait for hair to grow back, some horses show immediate improvement and other horses may take weeks, months or years before the hair grows back.