In Greek mythology, Leo was identified as the Nemean Lion which was killed by Hercules during one of his twelve labours, and next put into the sky.
The first of Heracles’ twelve labors, set by King Eurystheus (his cousin) was to slay the Nemean lion.
According to one version of the myth, the Nemean lion took women as hostages to its lair in a cave near Nemea, luring warriors from nearby towns to save the damsel in distress. After entering the cave, the warrior would see the woman (usually feigning injury) and rush to her side. Once he was close, the woman would turn into a lion and kill the warrior, devouring his remains and giving the bones to Hades.
Heracles wandered the area until he came to the town of Cleonae. There he met a boy who said that if Heracles slew the Nemean lion and returned alive within 30 days, the town would sacrifice a lion to Zeus; but if he did not return within 30 days or he died, the boy would sacrifice himself to Zeus. Another version claims that he met Molorchos, a shepherd who had lost his son to the lion, saying that if he came back within 30 days, a ram would be sacrificed to Zeus. If he did not return within 30 days, it would be sacrificed to the dead Heracles as a mourning offering.
While searching for the lion, Heracles fetched some arrows to use against it, not knowing that its golden fur was impenetrable; when he found and shot the lion and firing at it with his bow, he discovered the fur’s protective property when the arrow bounced harmlessly off the creature’s thigh. After some time, Heracles made the lion return to his cave. The cave had two entrances, one of which Heracles blocked; he then entered the other. In those dark and close quarters, Heracles stunned the beast with his club and, using his immense strength, strangled it to death. During the fight the lion bit off one of his fingers. Others say that he shot arrows at it, eventually shooting it in the unarmored mouth.
After slaying the lion, he tried to skin it with a knife from his belt, but failed. He then tried sharpening the knife with a stone and even tried with the stone itself. Finally, Athena, noticing the hero’s plight, told Heracles to use one of the lion’s own claws to skin the pelt.
When he returned on the thirtieth day carrying the carcass of the lion on his shoulders, King Eurystheus was amazed and terrified. Eurystheus forbade him ever again to enter the city; in future he was to display the fruits of his labours outside the city gates. Eurystheus warned him that the tasks set for him would become increasingly difficult. He then sent Heracles off to complete his next quest, which was to destroy the Lernaean hydra.
The Nemean lion’s coat was impervious to the elements and all but the most powerful weapons. Others say that Heracles’ armor was, in fact, the hide of the lion of Cithaeron.