Abstract: A harness apparatus for mounting a device on the outside of the tail of an animal, comprising a girth belt constructed to surround the body of the animal near the front legs thereof, the girth belt having a rearwardly extending belt portion positioned to be located near the spinal portion of the animal. A tail harness comprising a first flexible and resilient cord is connected at its forward end to the rear end of the girth belt portion. A second flexible and resilient cord having a first loop at its front end is connected to the rear end of the first cord, the second cord having two cord portions that extend rearwardly from the first loop and are constructed to extend around and under the tail of the animal. An adjustment device is mounted on the second cord portions near the rear end of the first loop to maintain them in closely adjacent relation. The adjustment device is slidable along the second cord portions and is positioned to be located in front of the tail of the animal.