I'm new here and new to horses. I've been enjoying this website quite a bit!
I agree with barefoothooves, it looks like Rojo has mange. I've seen and treated a lot of mange cases in dogs and this looks and sounds just like it.
Ivermectin is one treatment along with steriods to help with itching and inflamation. Also there is a great shampoo, Micro Tec made by Equiss. They also have a great spray that will sooth the skin. (I used this shampoo and spray on my horse when she started to get rain rot and it totally cleared it up iwth only one bath!)
Best thing to do first is have your vet do several skin scrappings.
And do some allergy tests.
If it's gnats, flys, mosquitoes or any other bugs the best spray I've ever used for me, the dogs and the horse is Bugrepel. You can find this on www.bugrepel.com
It's an awesome product! Plus it is totally chemical free and pesticide free. There is a lot of information on that website about that product.
Hope this helps!