I can't make heads or .... tails.... of what you are doing, but if I had to guess, I'd say you have applied a plane of alpha layered tail hair on top of an otherwise solid tail segment. When you turn the horse around, the hair plane does not show on the other side because in NWN, planes are not naturally double sided. To remedy this, duplicate the plane and flip its normals. Alternately, mirror the plane over whatever axis the horse points from head to tail, normally Y. If that doesn't help, post the model so we can see what is really going on.