Peppermints..

I kinda wanted to buy some. But I could've find any sales that seemed like a good deal.

And with that, what do peppermints do?

Peppermints act like any other treat, however the amount of stats gained from them is quite a bit.

There is a downside to them, which is they increase random skills. So using them all the time can actually cause a horse to do poorly in shows as it boosts their unnecessary stats.
Only way to fully take advantage of them and a horse still do well in the show ring is to set the horse's discipline to one thing while using peppermints, and then change the discipline to what you actually want later, returning to the use of discipline specific treats.

Besides buying them, you can find occasionally find peppermints at the wishing well. They just come up pretty rarely.