About four thousand years ago, Huangdi, the first legendary ruler of China, was traveling to the countryside with his entourage to visit an old friend. On the way, they encountered a boy watching over a herd of horses.
Huangdi asked the boy, "Do you know the way to my friend's village, which is far from here?"
The boy replied that he did.
The Emperor then asked, "Do you know my friend?"
Again, the boy answered yes.
Impressed by the boy's apparent broad-mindedness, Huangdi asked him, "Do you know how to rule a country?"
The boy said, "Ruling a country is not much different from watching over a herd of horses. You simply have to drive the wild horses out of the herd."
Huangdi left, deeply brooding over the boy's words.
This idiom originates from the boy's statement and is used to describe someone who has a bad influence on their peers.