What is unique about Sonepur Mela held in Patna?

Some of the unique features of the Sonepur Mela are;

This is the place where the freedom fighter Veer Kunwar Singh started to recruit volunteers to help him in the movement. The British utterly disliked this place. The very idea of cow protection in India started from this place. Sonepur mela is the biggest animal fair in Asia. In fact, its the world's largest cattle fair. The mela is the very place where Chandragupta used to buy horses and elephants. The fair lasts 21 days. You have a famous temple called the Harihar Nath temple, a sort of pilgrimage site for many as there is some sort of myth and story attached to this place. In addition to animals, you can by exotic plants, pottery, Madhubani paintings etc. 

I have good memories about this place. Been there many times as a child. I have written an article on the Sonepur Mela here. Feel free to read it. 

Hope this helps.