The Bodhisatta was once a snake-
The story was told in reference to a vow taken by Sāriputta. Some villagers once brought some meal cakes to the monastery, and when the monks present there had eaten, it was suggested that what remained should be saved for those monks who were absent in the village. This was done, but a young colleague of Sāriputta, arriving very late, found that Sāriputta had already eaten his portion, whereat he was very disappointed. Sāriputta immediately vowed never again to touch meal cakes, and the Buddha said that Sāriputta would never return to anything that he had once renounced.
The snake is identified with Sāriputta. J.i.310 f.