Step 2. Color in the Mane, Tail, and Forelock in the color you are using. Use the Normal Mode, 100 Opacity, and Scale 1. This is a large picture, I'm using Circle (19), but for a smaller image, use a smaller brush.
Step 3. Using many different shades and tints of the main color, use Brush size 3 to stroke "hairs" in the mane and tail. Go from nearly black to almost white.
Step 4. Here's where things start looking a little strange. Don't worry. We'll fix it up later. Using the Smudge tool (finger), smudge everything together. Look at the picture to see what 'everything' is. Do NOT smudge the forelock, mane and tail together. Smudge the tail. Smudge the Mane. Smudge the forelock. Use Circle 19 with a rate of 50. Also, smudge it out. If your having trouble not smuding the horse, hide the layer while you smudge.
Step 5. Add white rings or stripes on your horse so that it always faces the light source. If you don't know what I'm talking about, take a flashlight or torch, a horse model, and shine the light at different angles on the horse. The areas that are lightest are constantly moving.
Step 6.Now, smudge everything together. If your tail looked thin before, the new layer of smudging makes it thicker and fuller. You can use dodge to lighten it or burn to darken it.