During the Middle Ages, horses were a symbol of wealth, power — and manliness. So, if you really, really, wanted to hurt a guy, you could hit him where it hurt the most — by cutting off his horse's tail.
In his article, "'Tails' of Masculinity: Knights, Clerics, and the Mutilation of Horses in Medieval England" (Speculum: October 2013), historian Andrew Miller writes that, "Few greater images of power, wealth, and manliness in the Middle Ages can be conjured than that of a mounted knight charging into battle or of a nobleman astride a magnificent steed, falcon at his wrist, leading a braying pack of hounds on the chase."
So, by removing the tail of a rival's horse, it served as a ritualistic and symbolic act that attacked their masculinity, with the horse's tail serving as a phallic symbol. It was a potent insult, one that took place in medieval England, Europe, and even the Arabic Middle East.
No doubt, as noted in Medievalists.net, the physical condition and appearance of a horse were of the utmost importance, "for a handsome horse complemented its rider." Accordingly, horses were bred to have long tails and manes. From Medievalists:
The Normans were...pre-occupied with their own manliness and were much more open to using punishments such as castration and blinding than other medieval Europeans as they saw it as a way of destroying and emasculating their enemies. The same notion applied when attacking the horse's tail – the perpetrators were symbolically attacking one's manhood. Miller notes that Thomas Aquinas even found this to be the case when he wrote: "Vengeance is wrought on dumb animals and any other irrational creatures, because in this way their owners are punished."
The most notorious example took place on Christmas Eve, 1170, when Robert de Broc and his men attacked a park owned by Thomas Becket, the archbishop of Canterbury, where they cut off the tail of one of the archbishop's horses. The animal was brought to Becket, who exclaimed, "a mare in my service has in contempt of my name had its tail cut off—as though I could be put to shame by the mutilation of a beast!" The archbishop then excommunicated Robert. A few days later, on December 29th, Robert de Broc led the four knights who came from Henry II's court to Canterbury, where those men would enter the cathedral and murder the archbishop.
In addition to cutting off the tail, an attacker might also cut off the ears or lips of a horse — or make a person kiss his horse's ass:
For example, in 1313 a group of townsmen from Canterbury attacked the archbishop's home, where they found and seized Richard Cristien, the archbishop's dean. According to the complaint made, these men took the dean and "put him on his horse with his face to the tail and inhumanely compelled him to hold the tail and ride, with songs and dances, through that town and afterwards cut off the tail, ears, and lips of his horse, cast the dean in a filthy place, carried away writings, muniments and some privileges of the archbishop in his custody, and prevented him from executing his office."
Ah, good times.
Much more at Medievalists.
Image: Master Francke, The Mocking of Saint Thomas Becket, ca. 1424. Panel from the Saint Thomas Altarpiece, Hamburg, Germany.