Horses eat hay, grass, grain...they're herbivores. But what kind of little extras do we give our horses when we're happy with them?
Horse cookies or horse biscuits can be bought at tack stores or baked in your own kitchen. They're generally carrot, apple, or peppermint flavored.
Horses, by the way, love peppermint. Adore it. It isn't horse catnip, but it might be horse chocolate. Other than specially made horse cookies, the kind of treats we give horses include:
1. Root vegetables, especially carrots. Horses also like turnips and swedes - in fact the old English hunt cure for boredom in stalled horses is a large turnip or swede suspended from the ceiling of the stall using baling twine.
2. Apples, including crab apples - crab apples are too bitter for the human palate, but horses like them fine. Because of their size, it's also safe for horses to eat apple cores/apple seeds, and many riders will take an apple with them on a long trail ride, eat the flesh and feed the horse the core.
3. Peppermint candy. It's safe for horses to eat hard candy (in fact, at Christmas, I often hand out candy canes). Soft candy such as toffee should not be given to horses as it can give them cavities. In Britain, Polo mints - the local equivalent of Lifesaver mints - are in the pocket of most horsey teenagers.
4. Watermelon. On a hot day, horses appreciate watermelon - and they will even eat the rind.
5. Grapes. I've given grapes to old horses with no teeth before. Unlike small animals, they can also eat raisins. Other small fruits and berries are fine too.
Horses, like dogs, should not be given chocolate. They also shouldn't be given dairy - horses are very lactose intolerant.
Another thing horses like: Beer. (And it's almost impossible to get them drunk).
For worldbuilding, of course, you can always invent a fruit or herb horses are particularly fond of.