It's tempting to think of fur as just layers of hair which sounds easy to draw. Yet many artists struggle with it to the point they may just avoid it altogether. Below are some tips to help you draw realistic animal fur:
When drawing fur, it’s important to remember fur isn’t made up of individual strands of hairs, but rather the shadows between the hairs. Like an outline of a person’s face, what you end up with will be much different than what you intended.
Animal fur is three-dimensional, not two. Shade one side of the animal to give it this effect. Animal hair doesn't mat down perfectly. Draw hair coming from all sides. Each of the curves you draw will define the direction of the fur all over the tail. Animal hair clumps together. Try to keep the hairs short, with wide shadows, and avoid sharp, thin lines
Hairs will be thicker and wider closer to parts of the body such as the tail, but closer to the edge, they'll be narrower and tighter.
Use shadow to add dimension and draw bigger, wider shadows toward the base.
Use a blending stump or the end of a cotton swab to blend the clumps, tracing over the fur in the direction of its flow.
Remember, fur is nothing more than shadows and highlights. Your eraser comes in handy here, particularly on the illuminated right side.
Add tiny shadows within the clumps of hair. Be careful not to overdo these darker shadows.
Do some final inspections. Drawing fur is challenging and practice is truly the only way master its complexity.