Are you positive your pony has mites? I would get it diagnosed before doing any treatment. You didn't mention what your pony's symptoms are, but I can tell you from experience that a variety of things can cause a pony to itch, develop bald spots, get irritated skin, etc.
This winter my mini pony began to scratch furiously and actually rubbed parts of her winter coat almost bald! I did alot of research and can pass on what I learned. First, you should really find out if your pony has mites, lice, allergies, other parasites, rain rot or other skin dermatitis. Your vet can examine the pony, and take skin scrapings if he has doubts.
There are several types of lice. Some are visible under the mane, others live in the general body hair. If you spot lice then you can buy Louse Dusting powder from your vet. You will need to redust a few weeks later, and keep your pony away from others until the lice are under control.
Mites are not visible to the eye, that's why your vet will require a skin scrapping. They can be treated by giving Ivermectin wormer. Once again, I would ask my vet's advice as the pony probably will require two treatments of ivermectin before being cleared up.
If you do a search on the internet you can find alot of information on mites, lice, etc so once you know what you're dealing with you probably will be okay. I certainly would avoid using any really harsh chemical if there is a milder way to treat your pony!