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Juust saying, tails are basically and universally an extension of the spinal column. Horses have short tails ending in long hairs, and even birds have them in the form of a Pygostyle.
The rectum is universally under the tail for cleanliness reasons, and tends to be right up under the spine- it would be very difficult for the spine to go down between the pelvis and not cause posture-related issues.
