Let's say you want to draw a cat in Illustrator (who wouldn't want to do that?), how do you make the fur?
Here is a few tutorials on how to do so.
This one is drawing fur for a stuffed animal bear. Doesn't look ultra realistic, but you might be able to modify it to make it look more realistic if that is your desire. It is well written.
These are Photoshop tutorials, but you should be able to apply most/some of the techniques to Illustrator.
http://ballistic-kitten.deviantart.com/ ... l-22997085
http://shadowumbre.deviantart.com/art/F ... -104939086
http://fatal-mantis85.deviantart.com/ar ... l-25125510
http://yellow-eyes.deviantart.com/art/W ... D-32253741
http://venator-somniorum.deviantart.com ... l-36338777
Here is one on drawing hair in Illustrator. What is fur after all?
Here is a link to create "cute" looking fur.