Bull symbolizes strength and fertility in Hinduism. The cow here represents a fruitful earth while the bull is the fertile sky. The Rig-Veda associated the bull to a heavenly creature that fertilized the earth with his sperm. This bull was known as bull Rudra. They called their god Agni mighty bull while the bull-god Vrishabha was known to be responsible for the spinning of the cosmic wheel. The Nandi bull symbolizes sexual energy when ridden by Shiva but this energy is transformed at times to spiritual energy.