The small diameter fibre optics strands would be bundled tightly together and flush at one end (the end closest to the "horse") with a light source (obviously battery powered - either LEDs or incandescent bulb) at that end.  The light would seem diminished along the tail except at the very ends, which would glow brightly.  If you distorted the surface of the fibre optic strands (maybe pull each strand through a fold of sandpaper?) you would see some of the light all along the tail.