Step 25: Draw the shape of the horse's tail around the guide line on the far left side. Just like with the mane, draw a series of curved shapes for the clumps of hair on the tail flowing in the wind.
Step 26: For a cleaner look, erase as much as you can of the initial guide lines. Don't worry about erasing all of the guides. It's okay to leave some behind. Re-draw any final sketch lines you may have accidentally erased.
Final Step: Add some shading to your running horse drawing for extra detail. Horses can have different coat patterns, so you can shade yours however you'd like! For this look, shade the body using a medium value except for areas that are white, like a stripe on the head. Use a dark value for the horse's mane and tail so that they stand out from the body. As you shade the mane and the tail, leave a few sections blank to represent shine. For a more detailed guide on how to shade, check out this tutorial: How to shade.
Also use dark values to create shadows. This gives the figure more dimension and volume. Pick the direction of the light source when shading so that the shadows are consistent with it. Vary the pressure on your pencil to get different degrees of tonal value. Add the value lightly at first and gradually increase the darkness to a level that you like.
For a white horse, skip the medium value and just add a few shadows throughout the body. Continue slowly adding value until the entire drawing is shaded how you like. Shading can be time-consuming, so be patient and take breaks.
It's a good idea to use reference as you draw for a more accurate depiction of a running horse. Don't forget to pause the video to draw at your own pace. Add a shadow and some dirt being kicked up at the bottom to emphasize the running.