According to Indian mythology, there was a king called Puranjaya who belonged to the Solar dynasty (descendants of the Sun-God). In ancient times, wars between devas (gods) and asuras (demons) would continue unabated. During one such war, the devas went to Vaikuntha to worship Lord Vishnu and seek his assistance in the fight with the asuras, who had become a menace for all.
Pleased with their devotion, Lord Vishnu blessed them, “Go to earth and seek the blessings of King Puranjaya. He is a very brave warrior and he will agree to help you. Further, I shall enter his body to impart super strength to him by which he will earn victory for you.”
The devas then decided to approach King Puranjaya. They pleaded, “We need your help desperately. We have heard of your courage and valour, so we have come to seek your assistance.”
King Puranjaya listened to them attentively and after pondering for some time, replied, “I will help you but on one condition. The greatest among you all, Lord Indra, must carry me on his shoulders in your battle with the asuras. Only if he agrees to do so, will I consent to fight in the war and help you win it.”
The devas rushed to Lord Indra to beg, “If we have to overcome the asuras, you will have to agree to carry King Puranjaya on your back.”
Harassed by the asuras, Lord Indra readily agreed and took the form of a majestic bull. The devas with this bull reached King Puranjaya’s court to ask him to join them in the war. On seeing the bull, King Puranjaya mounted it and entered the battlefield to fight the asuras with full might. Lord Vishnu had meanwhile entered King Puranjaya’s body as promised and thus imparted him extra strength.
The war resulted in victory for the devas. As King Puranjaya had fought the battle astride the bull, he came to be known as Kakustha. In Sanskrit language, kakuth means the ‘back of a bull’.