The story of Vrishabha flag comes in the Mahabharata Anushashan Parva. Once the calf of mother cow Surbhi was drinking milk. While drinking milk, the calf got foam on its face. The same foam flew through the air and fell on Lord Shankar’s head. When this happened, Lord Shiva became angry.
At that time, Prajapati ji said to Lord Shankar, “O Lord! The splash of milk that has fallen on your head is nectar. Cow’s milk does not become false if the calves drink it. Just as the moon collects nectar and showers it, similarly the Rohini cows also shower the milk produced from nectar.”
Just as air, fire, gold, sea and the nectar drunk by the gods never become impure, similarly a cow feeding its calves also never becomes impure. This mother cow nourishes the entire world with her milk and ghee. Everyone desires the opulence of the nectar-like, sacred milk of these cows.”
After that, Prajapati ji gave many cows and a bull to Mahadev ji. Shivaji became happy and made Vrishabha i.e. bull his vehicle and decorated his flag with the symbol of the same Vrishabha. Due to this, he got the name ‘Vrishabh Dhwaja’. Then the gods made Mahadev ji the master of the animals and he was named ‘Vrishank’ among the cows.