Because it's Halloween...and you might just have difficulty resisting sharing with a horse.
You can feed some candy to horses. Hard candy is good, soft chewy candy that sticks to their teeth is not good (horses can get cavities). Their favorite flavor is generally (but not always) peppermint.
They should not be fed chocolate - they cannot metabolize theobromine - a tiny bit probably wouldn't hurt (theobromine also causes false positives on drug tests for show horses) - but a lot could certainly be poisonous for them.
However, the very best Halloween treat for a horse is...fresh pumpkin. Horses like pumpkins (I mean the standard orange ones here) and will eat the rind and seeds. (However, they should not be fed used jack o'lanterns because candle wax won't do them any good). Hanging a dessert or pie pumpkin from the stall roof is a great fall toy for a horse.
And, here's a Halloween costume - me as Ellen Ripley and Uptown Funk as the USS Sulaco.