Gypsy, the Equine Encyclopedia, has something to say about tail swishing, having mastered the technique herself.
Tail swishing is a way that horses show that they are annoyed or angry. It is most commonly used as a warning to other horses not to come too close. If this does not work, biting and kicking may follow.
When horses swish their tail while being ridden it is usually because their saddle is pinching, their back hurts or the rider is being too rough. Sometimes, however, there is no reason and the horse just enjoys doing it.
Horses also swish their tails to get rid of flies, gnats or other small insects.
Ponies often swish their tails when warning bigger horses not to get too close to their food.
Gypsy used to swish her tail so much that she earned the name “Swishy.”
The longer the tail, the better for swishing.