Nandi - Shiva's Escort

Nandi is the vahana (vehicle) of Shiva. As an independent deity, the bull was in ancient times, the Lord of Joy (Nandikeshvara), and was represented as a man with a bull's head. Joy, i.e., music and dance, were seen as the fundamental forces of creation, an aspect which was later transferred to Shiva. Later it was said of Nandikeshvara that he was a rishi (wise man) who guarded Shiva's door to become divine in this way.

Shaivite scholars state that "the original reason for associating the bull with this god was simple, the great masculine potency which each embodies, and the rage and physical power of which both are capable."

As the creature on which Shiva rides, Nandi embodies the permanent inner strength which can be acquired by controlling physical strength and violence (the name means: he who grants joy). Only those who have conquered desire and achieved self-knowledge can ride the bull like Shiva himself.

References:

Jansen, Eva Rudy. The Book of Hindu Imagery, The Gods and their Symbols: Holland, Binkey Kok Publishers, 1998.

Chatterjee, Gautam. Sacred Hindu Symbols: New Delhi, Abhinav Publications, 2001.