Bo Le was a famous master of horse-judging. He wrote a book titled Xiang Ma Jing (classics of how to judge a horse) to explain how to recognize a good horse. "A good horse has a wide forehead, bulging eyes and round hoofs," he wrote.
One day, according to a tale in the book, the son of Bo Le went out to look for a good horse. After a while, he came back, bringing with him a toad. He told his father, "I have found a horse similar that fits your description but its eyes are not bulging enough and hoofs not round." Bo Le did not know whether to laugh or to cry at this. He teased his son, "This horse is good at jumping rather than being ridden. What you have found matches my description, but it's not a good horse"
From what Bo Le said we draw the idiom, depicting those who work mechanically or try to locate something by following a description.