The horse holds 午, eleven to one — the brightest, highest-energy point of the day. 午 also sits directly opposite 子, so the horse stands diagonally across the system from the rat: one positioning in the dark, one running in the light.
You'll recognise it here: you need the sense of moving forward, and stopping is harder on you than tiredness; your feelings are visible and nobody has to guess; and what you like is the speed of the process rather than arrival — after arriving you feel oddly empty.
Running well, the horse is whoever pulls everyone along: your energy is contagious and a group simply moves faster with you in it. Blocked, it goes two ways. One is movement standing in for resolution — new job, new city, new relationship, the same problem against a different background. The other is depletion: peak energy isn't continuous, and you'll run until the engine cuts out before you learn to stop. The horse's practical lesson is scheduling rest rather than waiting to break. It isn't that you can't stop. It's that stopping was never part of the plan.