The Why

It's a bull (Nandi) with Shiva

Nandi (Sanskrit: नन्दि, Tamil: நந்தி, Kannada: ನಂದಿ, Telugu: న౦ది, Odia: ନନ୍ଦି) is the gate-guardian deity of Kailasa, the abode of Lord Shiva. He is usually depicted as a bull, which also serves as the mount to Shiva. According to Saivite siddhantic tradition, he is considered as the chief guru of eight disciples of Nandinatha Sampradaya, namely, Sanaka, Sanatana, Sanandana, Sanatkumara, Tirumular, Vyagrapada, Patanjali, and Sivayoga Muni, who were sent in eight different directions, to spread the wisdom of Shaivism.[2]The Cham Hindus of Vietnam believes that when they die, the Nandi will come and take their soul to the holy land of India from Vietnam.[3]

https://en.wikipedia.org/wiki/Nandi_(mythology)