This quote speaks volumes.
Why are we against this docking and nicking? The horse's tail is nature's instrument to remove flies and to give the hind parts of the animal warmth. On the forward parts of the animal there is provided by nature a fly muscle, and the animal has the power to twitch this muscle when flies alight on its skin and thus remove them; but the fly muscle does not extend to the hind quarters, and that is why nature gives the animal a tail so that it can swing it and scare the flies off its hind quarters and buttocks. When a horse is docked, normally at least four inches of its tail are removed, and it is from the last four inches of the tail that the long hairs grow. Shorter hairs grow higher up the tail. When the poor brute is docked, it can never get a long tail with which to flick away the flies.
This site contains the entire text of the argument presented to Parliament, demanding that this cruel, STUPID procedure, be banned. https://api.parliament.uk/historic-hansard/commons/1949/mar/04/docking-and-nicking-of-horses-bill
The argument goes on for several pages, citing so many examples and reasons why this kind of cruelty needs to be stopped. Their statements are compelling, and, the veracity of the arguments ring so true, event today.
Shame on people who continue to mutilate their horses.