The ancient Xianbei tribal chief Shegui had two sons: the elder, Tuyuhun, and the younger, Ruoluowei. When Shegui died, Ruoluowei inherited his father's position, while Tuyuhun was given seven hundred herder households.
One day, the horses of Tuyuhun and Ruoluohui fought each other. Ruoluohui sent someone to say to Tuyuhun, "Father has already divided the family for us. Why don't you go farther away!"
Tuyuhun said to the man, "Horses are beasts. Blaming a person for a horse fight makes no sense. Since he said that, I will go ten thousand li away!" With that, he led his herders away from their homeland in Liaodong and headed west.
Ruo Luohui regretted driving his elder brother away, so he sent a minister to bring him back.
Tuyuhun said to the pursuing minister, "If you can drive my herd of horses back, I will return with you."
The minister's men desperately drove the Tuyuhun horse herd eastward for several hundred paces, when suddenly the horses let out long and short neighs, turned their heads, and stampeded wildly back to the west.
Tuyuhun refused to return, leading his herders westward to the grasslands of Qinghai, where they lived on meat and dairy, dwelled in felt tents, and formed their own tribe.
This great migration of the Tuyuhun tribe occurred during the late Western Jin Dynasty. The tribe gradually grew stronger and was eventually assimilated by the Han Chinese. By the Tang Dynasty, their descendant Nuohébo was married to a Tang princess and granted the title King of Qinghai.
The Tuyuhun once obtained several Persian mares and placed them in Qinghai Lake, where they bred a blue-white horse capable of "covering a thousand li a day," known as the "Qinghai Steed." Later, the idiom "a thousand li a day" came to describe extremely fast speed.
Source: *Book of Wei*, "Biography of Tuyuhun"
Meaning of the Idiom: Later, the Chinese idiom "日行千里" came to describe extremely fast speed.