I don't know the first thing about zbrush, but you could first try to draw a sideview/frontview yourself to get the proportions right. I guess it's best to get those correct before thinking about details too much. you'll end up messing up your details if you change too much basic stuff later on in the project.
btw: http://bit.ly/xL7KVd
As the others said, you should go back and work on the proportions. Best to do this at a lower subdivision. The area between his ribs and pelvis is currently too long and his legs are too short. Right now he looks like a wiener dog whereas in the concept it's much more elegant.
As well, study the anatomy of foxes, wolves and dogs, get an idea for how it should really work. The attachment of the front and back legs doesn't seem anatomically correct.
The legs are too thick, in the concept they're much more slender. Also there isn't much suggestion of anatomy. You should look up some anatomy diagram of dogs and work in the basic muscle forms and landmarks. Even if it is covered in fur it will be noticable. I also feel that you jumped at detailing a bit too early. Do you have Z4R2b? I think flaying with the new fiber mesh could give you some nice results, I'm personally not a fan of the way it looks with long hairs but I think it would look fine with shorter hair.
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Hope this helps, I might try a quick paint over later if I get time.
btw: http://bit.ly/xL7KVd
As well, study the anatomy of foxes, wolves and dogs, get an idea for how it should really work. The attachment of the front and back legs doesn't seem anatomically correct.
critics please