To understand this, one must become acquainted with the symbolism of having a “tail.”
Devils have tails. The horns, goatee, pitchfork, red color and tail of devils all represent the negative psycho-features of these symbolic creatures. A tail, since it is the extension of the spinal column, and represents the outermost or lowest point of the brain, symbolizes the promotion and development of the lowest corporeal/sensual nature of a mere worldly personality. Therefore, a tail on a devil represents an opposing direction in life to things more heavenward. (That’s why “halos” are placed over the head and opposite to tails.)
However, if an angel were to display a tail this would symbolize that his or her lowest level of mind was completely under the control of God’s divine influence. In this case, a tail would represent the outermost extension of spiritual principles.
Here is a quick look at the psycho-dynamics of this ancient symbolic language.
According to scientist/theologian Emanuel Swedenborg, the mythical Greek creature called Pegasus, symbolized an elevated human mind. Why? Because a horse does for its rider what the human understanding does for its mind – moves it more swiftly towards its goal. A “winged” horse therefore represents an elevated or spiritual understanding, which can carry the human mind to higher levels of thought.
Pegasus, in spite of its noble and heavenly attributes, keeps its tail.
An angel with a “tail” represents a man or woman who was able to bring heavenly principles down into their very lowest terrestrial activities. Swedenborg stated that it is a rare occurrence at this day for individuals to have their lowest or sensual level regenerated by God—simply because it is so dominated by material and worldly ideas. (Arcana Coelestia 7442:4)
Therefore, an angel in heaven—seen with a tail—is a highly evolved creature! Such a creature represents a life where first and last things make one!