Step 20: Darken the line on the left for the mule's tail. Add a few short strokes at the bottom for loose hair. On top of the neck, draw a curved line that's made up of short strokes for the mule's mane.
Step 21: 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 mule drawing for extra detail. Mules can have different coat patterns. For this look, use a medium value all over the body. Use a lighter value on the muzzle and on some streaks across the body for highlight and sheen. Vary the pressure on your pencil to get different degrees of tonal value. Use a dark value to shade the mule's tail, the mane and the bottom part of the legs. Shading can be time-consuming, so be patient and take breaks. For a more detailed guide on how to shade, check out this tutorial: How to shade.
Add shadows to your mule drawing to give it more dimension and volume. Pick the direction of the light source when shading so that the shadows are consistent with it. Add the shadows lightly at first and gradually increase the darkness to a level that you like. It's a good idea to use reference as you draw for a more accurate depiction of a mule. Add some darker shadows on the neck and legs to emphasize the muscle structure.
Add a cast shadow underneath. This helps to ground the mule so it doesn't appear to be floating. Don't forget to pause the video after each step to draw at your own pace.