There are many magickal practices involving the merging of ones soul, or rather, astral body with their spirit animal, or an animal of their choice. It is not unheard of, and a lot of native american tribes believed in this. As far as physically, you could always do plastic surgery. Im sure doctors can be very experimental and creative. Sometime they may find a way for some people to get dogtails that work and are physically part of the body.