The Bodhisatta was once a woodpecker. One day he saw a lion suffering and asked what ailed it. The lion answered that a bone was stuck in its throat causing him pain and preventing him from eating anything. After the lion promised not to eat him, the Bodhisatta agreed to help. But not entirely trusting the lion, he propped his mouth open with a stick for added safety before poking his head in to dislodge the bone. Then he kicked out the stick and flew away into a tree without a word of thanks from the lion.
Sometime later the Bodhisatta saw the lion eating a wild buffalo. He flew onto a tree branch nearby and asked if he could get something for saving the lion’s life. The lion told him that not eating him when he had the chance was sufficient reward.
In the Lifetime of the Buddha
The lion was an earlier birth of Devadatta, a disciple of the Buddha who became his nemesis. While discussing Devadatta’s ingratitude one time, the Buddha told this story so people knew Devadatta had also been ungrateful in the past.