I need to ask a few questions in order to help:
1): What is the age of your horse?
2): Where are you located, and what is the weather like?
3): Is your mare eating and drinking normally?
4): Does she seem itchy?
5): How long has this condition been present?
6): Are you seeing any redness, crusting, oozing or anything else unusual at the skin where the hair has come missing?
7): What are the size and shapes of the hair loss areas?
8): When did you last deworm the horse, and what product did you use?
9): What is her current diet?
I realize these are a lot of questions, but this will give me more informatioon than I could get in a nine minute chat!
I'm looking forward to hearing from you.
As there are no abnormalities associated with the skin at the hair loss sites, it is most likely that what you are seeing is nothing more than a heavy shed of her winter coat. Normally, if an infectious/fungal/parasitic or auto-immune problem is present, there will be notable problems in the skin itself, which can provide clues to the condition. I would not totally rule out external parasite such as lice, but one deworming with an ivermectin-based dewormer on your part will take care of any lice that may be part of the problem. Other than that, I recommend observation only. Should you start to notice any redness, swelling, crusting or other abnormality in the skin, please contact your vet for a thorough exam and possible skin samples (biopsies) to obtain a diagnosis.
I've included a link for you which discusses many types of skin diseases in the horse. Don't be overwhelmed! As I stated above, as there are no skin abnormalities I wouldn't be overly concerned at this point.
I trust this has helped, and please let me know if you have any further questions.
Thanks, XXXXX XXXXX of luck!