During the 300s BC, when the Mauryan Empire was first uniting northern India, Buddhist monks and nuns in India began to tell stories about the earlier lives of the Buddha before he was reborn as the Buddha. Alphabetic writing was still pretty new and exciting in India at this time, and the Buddhists used the new alphabet to write down their stories as poems.
Many of these stories were not actually about the Buddha as a person. Many were short lessons in how to get along in the world. The story of The Monkey and the Crocodile, for instance, is a warning not to trust outsiders. The story of The Crab and the Crane shows that if you trick other people, they will stop trusting you. We call all of these stories the Jataka Tales.