A horse's tail is in many ways more important, and more expressive, than the tails of some other animals.
Anatomists say the tail is an extension of the horse's spinal column, and, indeed, it contains a series of vertebrae -- up to 18 or more. Horse tails also consist of muscles and their ligaments, arteries, skin and hair.
The hair of a horse's tail is interesting. It is made of keratin, and is closer in substance to our fingernails (or to its own hooves) than it is to the hair on our head. A strand of horse tail hair contains three distinct layers; the middle layer governs whether the hair curls or is straight -- this, in turn, is mainly dictated by the breed of horse.
Like that of the mane, a horse's tail is capable of being greatly enhanced in appearance by its owner. (source:Squidoo.com
A horse's tail is very important to him. Not only does it act as a fly swatter in summer, it keeps his "bottom" warm in winter! The area between a horse's hind legs is the only part not kept warm by hair, so it loses the greatest amount of warmth if left exposed by too much trimming.
From a horse's point of view, the best tail is a natural tail.
Some horses have thin ragged tails, while others have tails so thick they cannot be braided for formal riding events and shows. To make the best of their appearance, thick or thin, different tails are treated in different ways. Some styles even go in and out of fashion! (source: horsefun.com)