I think it might be her blanket(s)
there’s no scab or anything , it’s just her fur is a little darker but the flies have been getting to her so maybe it’s just her biting , thank you :)
It’s probably just a rub, but it could also be a skin condition. Are there bumps? Is it pink/white/discoloured? Is it scaly, crusty, or hard? Where specifically is it on your pony? If there’s nothing abnormal about it, other than it being a bald spot, and as long as your pony isn’t biting at it/excessively itching/ showing any signs of pain when you touch it it’s probably nothing major, but you should have someone look it over if you’re unsure.