In the reign of Emperor the Second of the Qin Dynasty (221-207 B.C.), the prime minister Zhao Gao, obsessed with ambitions, was planning to usurp the throne day and night. But he did not know how many of the ministers in the court were allowed to be ordered about by him and how many of them were his opponents. So he thought out a way to test how high his prestige among the ministers was and also to find out who dared to oppose him. 

One day when court was held, Zhao Gao let someone bring a stag to the court and, with a broad smile on his face, he said to Emperor the Second of the Qin Dynasty: "Your Majesty, here is a fine horse I'm presenting to you."  Looking at the animal, Emperor the Second thought that it was obviously a stag and that it couldn't be a horse. So he said smilingly to Zhao Gao: "Mister Prime Minister, you are wrong. This is a stay. Why do you say it is a horse?"  Remaining calm, Zhao Gao said: "Will your Majesty please see more clearly? This really is a horse that covers a thousand li a day." Filled with suspicion, Emperor the Second looked at the stag again and said: "How can the antlers be grown on the head of a horse?" Turning around and pointing his finger at the ministers, Zhao Gao said in a loud voice: "if our Majesty do not believe me, you can ask the ministers." 

The nonsense of Zhao Gao made the ministers totally at a lose, and they whispered to themselves: What tricks was Zhao Gao playing? Was it not obvious whether it was a stag or a horse? But when they saw the sinister smile on Zhao Gao's face and his two rolling eyes which were gazing at each of them, they suddenly understood his evil intentions. 

Some of the ministers who were timid and yet had a sense of righteousness did not dare to say anything, because to tell lies would make their conscience uneasy and to tell the truth would mean that they would be persecuted by Zhao Gao later. Some ministers with a sense of justice persisted that it was a stag and not a horse. There were still some crafty and fawning ministers who followed Zhao Gao closely in ordinary times. They immediately voiced their support to Zhao Gao, saying to the emperor:"This really in a horse that covers a thousand li a day."

After the event, Zhao Gao punished by various means those ministers with a sense of justice who were not obedient to him, even with whole families of some of those ministers executed. 

This story appears in "The Life of the First Emperor of the Qin Dynasty" in The Historical Records written by Sima Qian. From this story people have derived the set phrase "calling a stag a horse" to mean deliberately misrepresenting some thing and misleading the public.