Originally Posted by smrobs
Some horses do start shedding aggressively before their full summer coat has come up underneath and as a result, have little bald patches where the winter hair comes out but the summer hair hasn't come in yet.
At least, that was my first gut reaction, it could entirely be caused by something else like a fungal or bacterial infection.
Can you get some good pictures of the area you are talking about?
Yes. As long as the skin looks healthy it's nothing to worry about. This year both our sorrel mares are shedding in patches probably because of the very warm March.