The mithun is a type of domesticated bovine found in the Himalayan foothills of South/Southeast Asia. It is said to be a cross breed of a cow and a water buffalo. [Whoa!]

In Bhutan, mithuns are a distinct semi-wild cattle breed in themselves, and are indigenous to Arunachal Pradesh, Assam and Bangladesh.

This is all the information I could find on the term. I don't know in what context the term is being used, so I hope this helps.

Spivey