Nandi is the name of the gate- guardian deity of Kailasa, the abode of Lord Shiva. He is usually depicted as a bull which also serves as the mount (Sanskrit: Vahana) to the god Shiva. According to Saivite tradition, he is considered as the chief guru of eight disciples of Nandinatha Sampradaya - Sanaka, Sanatana, Sanandana, Sanatkumara, Tirumular, Vyagrapada, Patanjali and Sivayoga Muni who were send to eight directions to spread the wisdom of Shaivism
The incarnation of Nandikeshwar Suta says- Sage Shilad did a tremendous penance to please lord Shiva with an aspiration to have a son. Lord Shiva appeared before him and asked him to demand any boon he wished.
