Nandi is the vehicle for the Hindu god of Shiva. In Hindu mythology, Nandi is the bearer of truth and righteousness.
Each Hindu god has their own vahana (the Sanskrit root word for the English term of Wagon) that they use in war or in peacetime. These vehicles typically tend to be endangered species (such as eagle, peacock, swan, tiger, owl, lion) which Hindus want to protect from getting slaughtered. The animals also stand for specific qualities that are consistent with the deity's image & functions.