When it comes to drawing in pen, work on each section of their bodies more than the outline. Thanks to deviding the body up earlier, I create fur in different densities. For this, I layer lots of short lined strokes to look like individual strands of fur. The heavier densities of fur needs the lines made by your pen to be close together and slightly heavier marks. Less densities of fur needs to be done lightly.You can make the outlines look like fur too. Be careful around the eyes and the inner ears, these areas need to kept as light as possible.
My next sketch shows two hares boxing. Mad March hares is a favourite sign of spring for some people. This scene of hares boxing each other and leaping around is actually a female fending off a group of rather frisky males. They will keep chasing her until the fittest remaining shows he is worthy enough to mate with her. And you can see this at any time of year, not just spring.