Vrishabha, or Nandi, is the carrier of Lord Shiva. The word Vrishabha, or Vrushabha, has two meaning – the ordinary meaning is the Bull – the vahana of Lord Shiva. There is a hidden symbolism in Vrushabha. A shloka says ‘Vrsham Bhasyati iti Vrushabha – one who illumines right actions, their results and all dharmas. Vrshabha thus symbolizes ‘the best’ and the Vahana of Lord Shiva can be taken as the symbol of a realized soul or Jivanmukta.
Vrishabha is also symbolically related to Lord Shiva’s role of destructor. Only a realized soul can easily understand the significance of death. Death is the beginning of regeneration and it is only a minuscule part in the great cycle of birth and death.
On the concept of Vrishabha as the Vahana of Shiva, Swamini Saradapriyananda writes
Shiva’s destruction comes every now and then whenever a man’s life-span is over. But it is not the final end, because we are born again and again to exhaust the ‘Vasanas.’ How to attain the final salvation? The Vrishabha indicates how. Nirkuta (9-22) gives another derivative meaning for Vrishabha : ‘Atribrihati reta iti’ – roots out of the seed. When a Sadhak serves the Lord faithfully just as an ordinary bull serves his master day in and day out, without demanding or seeking anything for himself, then this very attitude sheds the seed of Vasanas and he will no more be born again.
The Vrishabha attitude is the carrier of the most auspicious of all, the release from the bondage of life.