The Great Basin buckaroos have many customs handed down throughout the years, and one such custom is the "war knot."
The war knot is also called the mustache knot. It is practical, in that it's purpose is to keep the tail out of the way during brandings, where it could easily get caught up in the rope, and also to keep the tail out of the mud or from getting tangled in brush. I know I'm not the only one that has looked back to see a large tumbleweed dragging from my horse's tail! The knot should always be taken out before the horse is returned to pasture to avoid the tail getting caught up on fencing or the like.
Here is an instructional from Martin Black for how to tie a war knot: