The position of a horse’s tail communicates what a horse is thinking and feeling. Some positions are symbolic of the original cause. The original cause of a high tail position is because a horse is accelerating. This results in the muscles of the tail lifting it up. When the horse decelerates the tail drops down.
Tail held high
A horse no longer has to accelerate to have his or her tail in a high position. It has now become symbolic of an acceleration and therefore means that the horse wants to race off and do something e.g. to initiate play with another horse. In other words it means something like, “Let’s dash off and play together”.
When the tail has dropped down while the horse is stationary it might mean something like, “I am tired and weak, I submit to you, you are the boss”. If a horse is particularly submissive to another he may tuck his tail against his backside like a submissive dog. Horses at rest and relaxed have tails that are in a rested position, hanging down.
When a horse’s tail sticks out behind the animal it might mean that the horse has become very aggressive or tense and the base of the tail has become stiff.
When sex is on the menu, the tails of both males and female horses are held high because they are excited. The female’s tail is also held to one side as an invitation for sex while also being raised.
A swishing movement of a tail during which it moves in various directions was originally caused by the need to get rid of flies. But this primaeval action has now evolved into a sign of frustration or confusion by a horse. The tail will move sideways then vertically and around in an arc. There are no flies but the action is symbolic of the presence of irritating flies.
A powerful tail-swish may be caused by an angry horse. The tail may be flicked high in the air and then slapped down hard. A kick may follow.
Electric shock whip
When horses are whipped with a device which discharges an electric shock to the horse it stiffens the base of his tail and swings it around in a rapid circle then lifts it high in the air and slaps it down hard on its backside. It happens very rapidly and indicates illegal behaviour by the rider (depending on the country).
The high stepping Tennessee Walkers are, Dr Morris says, often brutalised. Their tails are docked and a false tail fitted over the stump which is erect and which gives the impression that the horse is excited. On other occasions they may place a piece of ginger into the anus which creates a similar tail appearance.
Source: Desmond Morris’s Horsewatching.