Do you have a picture? It is unusual, although not impossible of course, to get mould just on the outside of a pony - its normally age marks/cancer or simply in ground dirt. If it is just on her leg there is no need to submerge the entire pony so you could simply soak her lower body in oxi and then follow that up with a good scrub.
As mentioned most chemicals will remove the coating or cause it to become cloudy so if there is no need for her head and eyes to encounter the chemical, then you would be best to simply avoid it altogether.