Hello Sally's owner
What your pony has is most likely a bacterial skiin infection caused by dermatophilus congolensis - commonly called rain rot or rain scald. It is a common conidtion especially in heavy coated horses and ponies who get wet coats - esp if there is skin abrasion - which could have happened by the blanket rubbing areas of the skin. The condition can be painful, due to the skin lesions, but is not itchy.
To treat this condition, shampoo your pony with an iodine or antimicrobial shampoo - check your horse or feed supply store - products vary by country/ region but your local feed or horse supply storre should have an appropriate shampoo. Chlorhexidine and benzoyl peroxide shampoos are effective. You may have to repeat this shampooing to get all teh affected areas cleaned up and scabs removed. The organism is anaerobic - which means it likes conditions with very low oxygen levels, so the treatment is aimed at gently removing loose hair and scabs so the affected skin exposed to the air. Be gentle and cautious when shampooing to remove scabs - your pony may not like it and you don't want to get hurt. Keep blankets off so the skin and coat are exposed to air. Keep the pony dry. The condition usually resolves nicely once the coat is shampooed, scabs are removed, and the pony or horse is kept dry. Sometimes an antibiotic injection is needed - so if it seems your pony's skin is not healing, you may need to call a vet to administer antibiotics. Usually this is to speed healing in a horse that is ridden - so if your pony is a companion pony, you likely can treat by shampooing and keeping the coat dry and avoid injected antibiotics.
In the future, to prevent the re-occurence, do not keep your pony blanketed continously - make sure the blanket is not rubbing, and make sure her coat is allowed to dry out if she gets wet. In rainy condition, be sure she has shelter so she doesn;t remain wet. It is also important to regularly groom your pony with clean brushes. Be sure to disinfect all the grooming tools, blankets, etc (a bleach solution will work) because the organism can remain in those items and reinfect your pony or infect another horse.
Also, I suggest you purchase a basic horse/ pony care book. It is wonderful that you are caring for this pony. You and the pony will feel much better if you become knowledgeable about how to care for a pony.
I hope this information is useful in resolving Sally's skin condition. Let me know how it goes.
Wishing you and Sally health and happiness!