The horse (말, mal) is read through speed and forward motion — where the ox represents fortune through slow, steady labor, the horse represents the same basic idea of earned progress but moving considerably faster.
A galloping horse is read as rapid advancement — success, a promotion, or a goal arriving sooner than expected. Riding a horse skillfully, in control of its direction, is read as being genuinely in command of your own advancement rather than being carried along by circumstance, a meaningfully different reading from being swept along by wind or water elsewhere in this tradition.
A horse running wild or out of control flips this reading — ambition or momentum that's outpacing your ability to manage it, sometimes read as a caution to slow down even amid real success. A horse arriving at your door or approaching you directly is read as news or an opportunity arriving quickly, without much lead time to prepare.
A horse that refuses to move, or appears sick or weak, is read as stalled progress — effort that isn't currently translating into forward motion, worth taking as a signal to look at what's actually blocking things rather than pushing harder in the same direction.