Chronus - the son of Gaia, goddess of the earth, and Uranus, god of heaven - who had in his time overthrown his father - had been predicted that he, too, would be dethroned by one of his children. He therefore devoured them as soon as they were born. His wife Rea with a ruse rescued Zeus, her last offspring, who was brought up, according to one version of the myth, by two nymphs. Since Chronus was pursuing them, Zeus saved them by turning them both into constellations - the Big Bear and Ursa Minor, the Small Bear, - he himself changing into the constellation of the Dragon.
But the figure of the dragon appears in other myths: in the myth of Jason the Dragon was the sleepless guardian of the golden fleece that Phrixus, grateful to the gods for saving him, had donated to Aeëtes, who kept it until Jason and the Argonauts succeeded in seizing it.
A dragon, son of Typhon and Echidna (or of Forcide and Ceto), assisted the Hesperides - the "Nymphs of the Sunset" daughters of the Night in Hesiod's Theogony - in looking after the garden where golden apples grew, that Gaia had donated to Hera on the occasion of her marriage with Zeus. They used to sing in choir at the sources from which the nectar of the gods flowed, and are connected to the myth of Hercules, who seeked from them the fruits of immortality, which had been promised to him by Hera.