Horse have been known to shed out excessively. I have owned 2 horses that one spring both of them shed out their winter coats even before their summer coats were fully grown in. That resulted in some large bald patches of skin on both of them. Thankfully I did not worry about finding a hair loss cure for them, I simply waited until their summer coat grew in. Sometimes their hormones just get a little out of sync with nature and bald patches appear.
When a horse starts to shed, most people don’t realize that the shorter summer fur is already grown and in place when the longer winter coat sheds out. I guess they think that the long hairs are breaking off to produce the summer coat. That is not true.
If your horse sheds out their winter coats excessively then simply wait a few weeks for their summer coat to grow in.