An Elephant Harness is a type of mob equipment that can be placed on adult Elephants and Mammoths in order to make them rideable and so you can add extra items to the Elephant, such as an Elephant Howdah, an Elephant Garment, a Chest Set, and a Mammoth Platform. It is created using Hide, Wool and Iron Ingots.
An Elephant Harness can be applied by right-clicking on an adult Elephant or Mammoth. It can be removed by right-clicking on the Elephant/Mammoth with Shears. Elephants will also drop Harnesses upon death, or when a Scroll of Freedom is used. If a player 'sneaks' near their Elephant ('shift key'), the Elephant will sit for a short time, allowing the player to sit on the Harness by right-clicking on it. To dismount an Elephant, press the shift key once again.
Note: If an Elephant Harness doesn't work on your tamed Elephant/Mammoth, it takes a while for an Elephant or Mammoth calf to grow into an adult (4-7 Minecraft days). If you try to place an Elephant Harness on a calf while it is still growing up, it will NOT work.