Hi, I'm Dr steve.
Hi, thank you for accepting my question. I am Katie
Skin conditions in horses can be very difficult to diagnose as you have seen. Unfortunately many skin conditions
can resemble each other. It sounds like yo jhave addressed mange and external parasite infestations and bacterial skin infections. Another condition that I see fairly commonly is ringworm, This is a fungal infection of the skin and while it can be in teh shape of a circle or many circles, it doesn't have to look like this and any skin condition that has crusty scales could be ringworm. The treatment involves treating the affected areas with a topical solution like maleseb or chlorhexidine. There also is an oral medicine called griseofulvin that can be used as well. Of course the first step is to confirm that ringworm is the problem. this can be done by taking a sample of hair and the crusts or scales and put it on a dermatophyte test culture medium. If it turns positive, then it is ringworm and your vet can give you more details about the treatment.
I hope this has helped. If it has would you please click the accept button so I may receive credit for my answer. Thank you and good luck. If you have more questions, please continue.