Hi,
You draw very well zorbels. I especially like how you've used colour pencils in the bulldog picture. The lion, ostrich and horse are nice too. I like the horse most of them, as it's of more complex perspective. Hmm, have you drawn these from a source, or from your head?
Both the line darkness and width count. In some cases it may be good to slightly erase the lines where the light hits the most. Eg. in the horse picture, the shape lines are pretty heavy. Remember that you don't have to encircle the shape with a line, instead some spots can be left open.