Buddha used to be a turtle in one of his previous lifetimes
A story that is often depicted in Buddhist art is the one where the Buddha in a previous life is a Bodhisattva-turtle (that is: a turtle who has vowed to save all sentient beings).
A group of merchants were sailing, when a storm hit. The giant Bodhisattva-turtle saw the ship wreck and saved the merchants by letting them climb on his back. He brought them safely to land. Tired from the long swim and heavy load he fell asleep on the sand.
The merchants were hungry and thirsty and after the ordeal were still not sure of their lives. They discussed amongst themselves how to find food. One of them said that they should kill and eat the turtle.
The Bodhisattva-turtle heard this and out of compassion for the merchants, decided to stay put and be eaten.
Jatakas are stories of previous lifetimes of the Buddha