Characters who are ghosts maybe be depicted as partly ephemeral to indicate this; one common form is for the character's legs or even their waist to terminate in a translucent, wavy or wispy form similar to popular depictions of spirits or ghosts in media. Compare mermaid for the variety of ways the legs may differ from a normal human's.
Such characters often have the ability to fly or hover, and as such do not require their legs for movement.
Characters with this trait often wear a long dress or robe, which enables them to appear human by hiding their abnormality.
Despite the name, very few such characters have what most people would call a "tail", since the feature stems from their body or legs rather than the base of their spine.