Nandi (bull) - in Hinduism

In Hinduism

1. A primary god: Nandi as a separate god can be traced back to Indus Valley Civilization, where dairy farming was the most important occupation, thus explaining the appearance of various artifacts, such as the 'Pasupati Seal,' indicating a deity much like Shiva. This deity- also known as Pasupati is believed to have been worshipped as the keeper of herds. Some puranas describe Nandi or Nandikeshvara as bull faced with a human body that resembles that of Shiva- in proportion and aspect, although with four hands, two hands holding the Parasu (the axe) and Mruga (the antelope) and the other two hands joined together in the Anjali(obeisance). Brahma Vaivarta Purana mentions Krishna himself to have taken the form of a bull as no one else in the Universe can bear Shiva.
2. Vehicle of Shiva: The bull Nandi is Shiva's primary vehicle and is the principal gana (follower) of Shiva.
3. Gate keeper of Shiva's abode: The close association of Shiva and Nandi explains the presence of a statue of Nandi at the gate of many temples dedicated to Shiva. It also explains why the word "nandi" in the Kannada, Telugu and Tamil languages is used as a metaphor for a person blocking the way. In Sanskrit, a bull is called "vrisha", which has another connotation - that of righteousness or Dharma. It is important to seek the blessings of Nandi before proceeding to worship Lord Shiva.
4. Chief in Shiva's army: Some Puranas mention that Nandi lead the Shiva Ganas, Shiva's attendants.
5. A Guru of Saivism: In addition to being his mount, Nandi is Shiva's foremost disciple. In the Natha/Siddhar tradition, Nandi is one of the primal gurus. He was the guru to Siddhar Thirumulanathar, Patanjalinathar and others.
6. From the yogic perspective, Nandi/Nandhi/ Nandikeshvara is the mind dedicated to Lord Siva, the Absolute. In other words, to understand and absorb Light, the 'experience and the wisdom' is Nandi which is the Guru within.