THE SEAL FOUND IN HARAPPA, another hint of existence of Shiva and Fertility worship in the Oldest Civilization of the World
Nandi means joyous. Nandi the Bull is the vahana (vehicle) of Shiva. 
The bull, a symbol of male sexuality, is the driving force of attainment in many symbolic systems. 
If, as we have seen, the cow stands for wealth, then the ox stands for work.