The fact that your 20 year old horse still has it’s winter coat in the summer in Southern Indiana would be a definite concern. It could be a symptom of a few things including Cushings Disease or even thyroid issues.
It is in your best interest to have her looked at by your veterinarian so they can run some tests and run a full blood screening. You should have this addressed right away.
Please let us know the outcome and good luck.