No. A peppermint candy as a treat is not an issue. As with any treat, there should always be limits particularly to ensure not promoting aggressive behavior. The quarter horse, Jody Grey, that I rode in my early childhood loved peppermints, especially around the holidays when they were more prevalent. However, when I was not paying attention cleaning his stall or standing next to him he would muzzle his way into my jacket’s kangaroo side pockets looking for that treat. Cute at first, but could be a nuisance over time.
Typical bite size peppermints are about two-thirds sugar which is a non-issue relative to the body weight of an adult horse and relative to the sugar and starch content of a typical diet.