The infographic covers the basics and a little more on horse tail movement signalling. There is some additional information below it.
Firstly, I am thankful to Desmond Morris PhD who is an author and zoologist.
Excitement and alertness
In general, the tail rises upwards in the words of Morris “like a needle on a dial” which neatly registers excitement. The tail held high indicates alertness as one would expect.
The origin of this raised tail position is that when a horse is running the tail moves upwards as “its anti-gravity muscle system goes into action”. When the horse decelerates the reverse happens. This is an interconnection between activity i.e. running/accelerating and the horse’s tail position. It evolved into being a signalling action by the tail.
Boisterous and willing to engage in activity
If a young horse approaching another wants to play, they may flick their tail high over their back. It may even curl over the back to initiate play. It is an invitation signal and understood by the other horse. Morris says that the tail movement is not caused by the acceleration of the horse but is symbolic of it.
Droopy tail
As mentioned above, the horse is less active and this has evolved into a signal to tell others that the horse does not want to be active and is feeling tired and weak. It may even signal that the horse is submitting to another. Sometimes horses place their tail between their legs as a sign of submission.
Aggressive
The tale is stiffened. The fleshy bit of the tail then sticks out behind the animal.
Sexual encounter
Both horses are excited and the tail is raised high because of their mood but there is a slight difference by the female with the tail held over to one side as an invitation.
Swishing movements
This as you would expect is to swish away irritating insects such as flies. But this has also evolved into a signal to another that the horse is perhaps anxious, confused or frustrated. It may be a signal that they are irritated if the tail follows an arc.
Particularly angry
This swishing movement can be exaggerated when a horse is particularly angry to the point where the hairs whistle through the air. It can be so vigorous that it can draw blood if it strikes human flesh.
