As per Linga Purana, Sage Shilada underwent severe penance to have a boon, A child with immortality. Lord Indra pleased of the austerities suggested him to pray to Lord Shiva who could provide the boon, nobody else could provide such a boon.

As per Linga Purana, Sage Shilada underwent severe penance to have a boon, A child with immortality. Lord Indra pleased of the austerities suggested him to pray to Lord Shiva who could provide the boon, nobody else could provide such a boon. He continued his penance for thousand years. He was totally immovable for many years, so the termites settled on his body and slowly started to build up their nest. The insects started to eat his flesh and imbibed his blood. Lord Shiva appeared and gave the boon for an immortal child. He himself will be born as his child. Sage Shilada performed Yagna and a child appeared from the sacrificial fire. The boy was named Nandi, who brings joy. By the age of seven the boy was well versed in Veda and all sacred texts. One day the two deities - Mitra and Varuna visited Sage Shilada. At the first sight of the boy, they commented : Though the boy had all auspicious signs, he would have a very short life. ‪So Nandi began to pray to Lord Shiva. Shiva appeared before him and blessed him. Lord Shiva adorned the boy with a necklace around his neck which he was wearing, and made him immortal. Lord Shiva blessed the child and declared that he would be worshipped along with Him . Immediately the boy got all the divine powers, three eyes and ten hands. Soon He was adopted as the child of Goddess Parvati.
Brahma Vaivarta Purana mentions Krishna himself to have taken the form of a bull as no one else in the Universe can bear Shiva. Nandi is Shiva's primary vehicle and is the principal gana of Shiva. It is important to seek the blessings of Nandi before proceeding to worship Shiva. Some Puranas mention that Nandi lead the Shiva Ganas, Shiva's attendants. In addition to being his mount, Nandi is Shiva's foremost disciple.
From the yogic perspective, Nandi/Nandhi/ Nandikeshvara is the mind dedicated to Lord Shiva, the Absolute. In other words, to understand and absorb Light, the 'experience and the wisdom' is Nandi which is the Guru within.
Om Nanikeshwaraya Namah ~ Shivaya Namah Om!