When attempting a realistic donkey drawing, I think of a miniature horse. However, unlike a horse, where you are focused on the muscular built, we will be illustrating fur more than anything else to mold it into a proper donkey drawing.
In terms of description, the mane is stiff and the tail is course. The body hair is short for most of the length while the legs are long and thin with tiny feet. They also have long ears and a relatively large head and neck in comparison to the rest of the body.
While there are many types of donkeys, the common one seems to have a grey coat with "light points" around the muzzle, eye, belly, and parts of the inner leg. Already, you can see just how much different a donkey is compared to a horse.
Given these descriptions, we will draw out the form and then filling it in with these light spots. After that, we will smudge out the fur and add in some simple shading to bring the donkey drawing to life.
In order to draw a donkey correctly, you need to start with a gesture sketch that focuses on proportions and form. It would be a good idea to look at many pictures of a donkey so you get a sense of the overall shapes involved.
Use basic geometric shapes to outline where the pivot points of the arms and legs are. Then, fill in the body and the head. Aside from the description, fill in where the bones are located. Each angle of the bones can help you further define the overall form.
Generally speaking, the entire animal can be summed up with simple rectangles, triangles, and circles. In terms of proportions, think about how many heads will comprise the body and then work from there.
Another sense of proportions is the square body and how the donkey stands. You will realize that the height of the body is half the length of a given limb. Use this information to give an accurate representation of your donkey drawing.
This section will require you to create a solid transparent object. First, fill in the outline created by your gesture drawing by creating a new layer underneath your gesture drawing and then fill it in with a dark base color.
As discussed, a realistic donkey drawing requires thinking about textures that define the animal. Once more, there are white patches around the eyes, the nose, limbs, and the underbelly. Use a chalk brush to block in these crucial areas.
Merge both layers together once you have some general shading patches of white areas done. From here, you should have a good idea of how the donkey drawing will shape up to look like.
Now it would be time to start working out the details. Specifically, the fur that needs to be drawn in first. The big picture in this step is that you will not be using your paint brush. Instead, you will use the smudge brush.
Smudging not only creates fur, but it also blends in the gesture lines very well while creating additional bands of fur. But make sure you use the chalk designed smudge brush for it. The jagged edges of the chalk head will create little filaments of fur from your gesture lines.
As you smudge, control it to smudge in the direction of the fur. For example, smudge gently around the outline of the donkey to get small simple furs sticking out. For the mane, smudge hard and in a vertical manner to make it stand up.
You need to do this for the white patches around the eyes, the belly, and the nose. Keep the pressure on the low end for these facial areas. As long as you're smudging in the proper direction, fur will naturally come out.
You may also switch back to your brush tool and block in additional colors or shapes to be smudged. Again, the goal here is to use your gesture lines and blocked shapes to create fur through controlled smudges.
When you have most of the fur done, it's time to add some light and shadows. First, you need to preserve the transparency of your donkey drawing. That way, any additional painting that you will need to do will not cross outside of the shape's boundaries.
There are many ways to do this step. If you are using Photoshop, create a new layer and turn it into a clipping mask. From there, set the transparency to 50% and work on the shadows.
If you are using something else, you can just duplicate the donkey layer, paint on top of it, and then adjust the layer's transparency to show the details of the previous donkey layer. Both methods will preserve the original donkey drawing layer.
The other method is just to paint right on the donkey layer if you're comfortable with your skills. To retain as much of the details as possible, opt to use the dodge and burn tools rather than the paint brush.
While not as effective as drawing lighting on a separate layer, this should allow you retain most of the details while still creating lightened and shadowed areas. Once you are done with that, add in a cast shadow.
You may also choose to sharpen the entire drawing to make the fur show up better. This can be done on a targeted area with the sharpen tool or it can be done globally with specially designed filters. The choices are up to you.