Hi there. Thanks for the question. Let me see if I can help. The most common cause of one sided nasal discharge in an older horse, is an abscessed tooth. I would highly recommend that you have a vet or equine dentist out to see your horse and do an oral exam to identify the problem tooth and remove it. This is your best course of action given the information you have given me. I hope this helps. Best of luck. Let me know if you have any further questions.