You could try just using showsheen with cowboy magic and try to get through it but the the girth area slippery
Metal curry comb. You could clip, however would look odd just the under part so would need to do the whole body. But, clipping season is over now.
We are sorry but we don’t have products who can help you for mud rash.
Did you already contact your vet ? He will helps you with a diagnostic and will give you appropriate drug if necessary.
We hope you will find a solution for your pony !
Have a nice day
Equistro Team !