The four Greeks Legends about how  the ancient Olympic games begun

The one Greek legend said that the great Herakles (Hercules, in the Roman form) won a race at Olympia, a plain in the small state of Elis, and then institute that the race should be re-enacted every four years.

The other one said  that Zeus himself had originated the festival after defeating Cronus for the sovereignty of heaven.

Another legend was that King Oeomaus, Hippodamia's father, challenged anyone who wished to marry his daughter to drive with the Hippodamia away in a chariot race against the King and  run faster in order not to be killed by the King Oeomaus' lance. Thirteen brave fiancées tried this feat, and they all failed resulting in death by the King Oeomaus' lance. The only one that managed not to be killed by the King Oeomaus' lance was Pelops. What he did was to bribe King Oeomaus' servant to rig the king's chariot with a faulty axle. As Pelops and Hippodamia were driving away, the King's chariot axle gave way and he fell to his death. Pelops then had a wedding celebration, including athletic contests, with his fellow Olympians. Supposedly, those contests were the first Olympics.

The last one legend of the games' origin includes references to Greek mythology. Hercules, the son of Zeus, was forced by Hera to kill his children. In order to redeem his honor, he had to serve King Eurestheus, his cousin, who commanded him to complete twelve labors. Hercules supposedly organized the Olympic Games to celebrate his completion of the fifth labor, cleaning all of Augeas' horse stalls in one day. By holding the festivities, he was thanking Zeus for his help in completing the labor.