The Horse Which Does Harm To The Herd - A Black Sheep

It was about four thousand years ago. Huangdi (Yellow Emperor)-the first legendary ruler in China went to the countryside to visit an old friend with his entourage. They met a boy keeping watch over a herd of horses on their way.

Huangdi asked the boy, "Do you know the way to my friend's village far away from here?"
The boy said yes.
Then the emperor asked, "Do you know my friend?"
The answer was yes again.
Huangdi thought the boy was broad-minded. So he asked him,"Do you know how to rule a country?"
The boy said, "There is little difference between ruling a country and watching over a herd of horses. You simply have to drive the wild horses out of the herd."
The emperor left, brooding over the boy's words.

The idiom "The horse which does harm to the herd - A black sheep" is taken from what the boy said and is used to describe anyone who has a bad influence on his peers.

害群之馬

大約四千年前，黃帝－傳說中中國的第一個帝王，帶著隨從去鄉村看望他的一個老朋友。路上，他們遇到了一個看守馬群的小男孩。

由於去朋友的村莊路途遙遠，黃帝就問小男孩：「你知不知道村莊的路？那兒住著我的一個朋友。」
男孩說知道。
黃帝又問男孩：「認不認識我的朋友？」
男孩回答認識。
黃帝想這小孩懂這麼多，於是問他：「你知道該怎樣治理國家嗎？」
男孩說：「治理國家的方法和看守一群馬差不多。只要把野馬從馬群裡趕出去就行了。」
黃帝離開時一直想著小男孩的話。

成語「害群之馬」出自於這個小男孩說的話，用來形容給集體帶來壞影響的人。