Why do the zodiac horses and monkeys have a fate?

After the Monkey King made a scene in the Heavenly Palace, he was sealed by the Jade Emperor and raised horses in the Heavenly Palace. Since then, horses and monkeys have formed an indissoluble bond. Sixty years ago, when I was in the countryside, I saw the performance of "Horse Racing and Selling" (country circus): the monkey official rode a horse. I saw the little monkey wearing an official hat and official clothes, riding on a horse, very proud.