The Bodhisatta was once a farmer. A traveling salesman used a donkey to carry his wares; and when it was not needed, he draped a lion skin over it and set it loose in barley and rice fields. People were usually too afraid to shoe the “lion” away, so they just let it eat their crops. When the salesman came to the Bodhisatta’s village, a watchman gave the alarm about a lion, and all the villagers came out shouting, blowing conchs, and beating drums. The donkey got scared and let out a hee-haw. The Bodhisatta then realized it was a donkey in disguise, and when he announced this, the villagers ran up and clubbed it to death. When the salesman returned, he remarked that his donkey was too stupid to keep its mouth shut.
In the Lifetime of the Buddha
The donkey was an earlier birth of a dimwitted disciple of the Buddha who was completely unaware how stupid he was. He had made it known to other disciples that someday he wanted to recite scripture at assembly, and he was invited to do it. That evening he took his seat in the jeweled pavilion in front of the others, and just as he was about to begin speaking, he grew nervous and ashamed. He got the first verse out, but could not remember the second, so he left and went to be alone in his quarters.
Later, when the Buddha heard some of his disciples discussing the incident, he told them this story so they knew that this empty disciple had also betrayed himself by speaking in the past.