I'm afraid people, it's not a tail. Have any of you ever seen the character sketches Rumiko Takahashi did for Sesshoumaru? There is a front, back and side view. At the back you can clearly see the "fluffy thing" is just draped over his shoulders, and doesn't behave like a tail at all (that is, it doesn't go into his lower back anywhere, it's just wound around his shoulders).
Submitted by Gingerfish