There is no substantial proof that rhinos were used. In 1616 Ulisse Aldrovandi published an illustration of a rhino wearing a collar, which hints at some degree of their possible domestication but doesn't support their use in combat. Similarly there have been depictions from the Indus valley of rhinos with ropes around their necks. While the famous Albrecht Durer print of the "armored" rhino might suggest it was some kind of "battle" rhino, Durer actually never saw the animal (which was a gift from India to the king of Portugal) and the "armor" is more indicative of his style as opposed to reality. I read some completely unsubstantiated stuff on multiple sites claiming the Ahom people fed alcohol to rhinos and sent them into battle like "tanks." I have yet to see any historical source for this and the section of wikipedia people seem to be getting that from has been removed. If anyone has a real source for this, please let me know because I'm very curious.

Long story short, there's no proof that they were ever used in battle.