I use MTG with great success also! In the past I have washed them in iodine shampoo and then if the rainrot was not ready to shed off the old dead skin, I would apply desitin diaper rash cream liberally to the area. Healed the skin and helped to make the old dead stuff come off, but it did make the hair all sticky and nasty till you could wash the horse again.
As someone else said, test the mtg somewhere not noticeable, some horses are allergic to it. One member on here had her friesians tail fall out completely from using it.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"