When a horse sees a spider it will start running at a speed no one ever thought a horse could go! Horses are super afraid of spiders, you see, so if they see one, they'll bolt and reach top speed! That's actually the only way to get a horse to run that fast.
Once people realize that they can increase the speed of their horses, soon (しゅん) everyone will be taking spiders with them to scare their horses into running faster. I say soon because it hasn't quite caught on yet. But if you help to spread the word it can happen even sooner.