The majority of horses love the taste of peppermint - and some will do pretty much anything for it.
Horses have a very fragile digestive system - and mint is very good for the digestive system. It's likely that they instinctively know it's good for them. In fact, if a horse is being picky about their feed, adding fresh mint will often get them to eat it (or mask the taste of medicine they don't want to eat).
As long as you don't give them too many, mint flavored candies are okay for your horse. (I jokingly call mint horse chocolate).