The Nemean Lion was a vicious monster in Greek mythology that lived in Nemea. It was eventually killed by Heracles. The lion was usually considered the offspring of Typhon and Echidna, but it was also said to have fallen from the moon, offspring of Zeus and Selene. A third origin has it being born of the Chimera.
The slaying of the lion
The first of Heracles' twelve labours was to slay the Nemean Lion and bring back its skin.
The lion had been terrorising the area around Nemea, and had a skin so thick that it was impenetrable to weapons. When Heracles first tackled it, using his bow-and-arrow, a club made from an olive tree he pulled out of the ground himself, and a bronze sword, all were ineffective. At last Heracles threw away his weapons and wrestled the lion to the ground, eventually killing it by thrusting his arm down its throat and choking it to death.
Heracles spent hours trying unsuccessfully to skin the lion, and gradually growing angrier as it appeared he would be unable to complete his first task. Eventually Athena, in the guise of an old crone, helped Heracles to realise that the best tools to cut the hide were the creature's own claws. Thus, with a little divine intervention, Heracles completed his first task.
Thereafter, he wore the impenetrable hide as armour. King Eurystheus, Heracles' taskmaster for the labours, was so frightened by Heracles' fearsome guise that he hid in a large bronze jar, and from that moment forth communicated all his instructions to Heracles through a herald.
The great lion of the constellation Leo was said by the Greeks to have been the Nemean Lion, placed in the sky after Heracles slew it. Alternatively, they said that the lion was cast down from the moon, which would also appear in this constellation during summer months. The sun appears to pass through its mouth, suggesting a method of death, and thus implying the difficulty in other methods.