Shiloh has been having the same problem. I tried tons of different salves and goops too didnt help. The only thing that I have found to help relieve her itching is regular baths with Alodine shampoo that I leave on for about 5 min. Before rinsing off. Diligent use of fly spray on her legs and belly in particular, and MTG rubbed into the effected area once a week. She isn't completely clear but it seems to have stopped her from itching so severly. I think its some sort of allergy since none of the other horses are affected.
The Vet thought it might be a grass allergy so I have mowed my pasture too. She's back on antihystamins and is getting better so maybe its a combination of things.
If its like Shilohs its only on the legs chest belly neck and a little on the jaws. She is clear from about the ribcage up. Nothing on her back upper neck hip or rump. I think that's why the grass allergy was diagnosed.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France