If a horse is a little pushy, feeding him peppermints may make him tend to nip, but that's true of any hand feeding. I've never heard of sugar giving a horse tooth decay - there's plenty of sugar in sweet feed, it's loaded with molasses. Most of the dietary supplements have a molasses base to make them palatable.
My mare doesn't like peppermints - she'll run them around in her mouth for awhile, then drop them on the floor.
What she likes is these:
She'll run over you for one if you're not careful. My dog likes them too. Mostly oatmeal, chopped carrots, and molasses.
Mmmmm, sounds yummy!
She LOVES her treats and gets annoyed if I give any to the other horses so I have to do it on the sly, LOL