According to some Puranas, Nandi was born to sage Shilada who got him by the grace of Shiva

According to some Puranas, Nandi was born to sage Shilada who got him by the grace of Shiva. It was Nandi who cursed Ravana that his kingdom would be burnt by a monkey (Vanara). And later Hanuman ji burnt Lanka when he went in search of Maa Sita, who was kept in Ashok Vatika.

According to one incident, Nandi took the form of a whale. At Mount Kailash when Lord Shiva was explaining the Vedas to his beloved wife, Parvati lost her concentration and Lord Shiva cursed her to be born as a fisherwoman. Later Lord Shiva repents for the mistake. Years later when Parvati reached her marriageable age, Nandi took the form of a whale and started to trouble the people. Later Parvati's father told that anyone who killed the whale will marry his daughter. Lord Shiva took the form of a fisherman and killed the whale. He married Parvati and returned to Mount Kailash happily.

In one puranic story, it is stated that once Shiva and Parvati were playing a game of dice. Shiva and Parvati agreed to have Nandi (the divine bull) as the umpire. Nandi is a favorite of Shiva. Although Shiva lost the game, Nandi declared him the winner. It is stated that Parvati was indignant over Nandi's partiality for Shiva and cursed him that he should die from an incurable disease. Thereupon Nandi fell at the feet of Parvati and pleaded for forgiveness.

"Mother forgive me. Should I not show at least this amount of gratitude to one who is my master? Is it not humiliating for me to declare that my master has lost the game? To uphold his honor I no doubt uttered a lie. But am I to be punished with such severity for so small an offence?"

Nandi prayed for forgiveness in this manner. Parvati forgave Nandi and taught him the means to atone for his lapse. She told him. "The Chaturdasi day in the month of Bhadrapada is the day when my son's birthday is celebrated. On that day you have to offer to my son what pleases you most (green grass)". This means that one atones for one's sins when one offers to the Lord what is most pleasing and enjoyable to him. For Nandi the most enjoyable and relishing food is green grass. As directed by Parvati, Nandi worshipped Ganapati by offering green grass. Nandi was then relieved of his dreaded disease. His health improved and by the grace of Parvati he was redeemed.! ✨🐂🕉🍂🌺🗿📿🏯🌿🙏🏻✨

Jai Shri Nandishwaraya Namah ~ Shivaya Namah Om 🌺🍂🌿✨🐂🕉🍂🌺🌿📿🌀✨🙏🏻