A sheet bend is used for joining two ropes. In this case, the tail is one rope.
This method modifies the standard Sheet Bend above to make it a Quick Release Knot. The Tail Tie is not used as a primary "handle" for moving a horse, is NEVER used with a tow vehicle, and, obviously, not with a horse with a docked tail. It can be used to guide a horse's path in a backwards drag, the tail must remain in line with the spine, and NEVER EVER tie to the actual tail bone. The knot is ALWAYS tied to HAIR ONLY. If you are not sure of where the tailbone ends do NOT use this knot.