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.