The biggest flaw I see here when you're trying to add fur is simple. You're trying to take a form and turn it
into fur, instead of imagining fur
on top of the muscles. You carefully molded out various muscle structures, and shade things according to them, only to try to turn them into a fur texture. For this reason I disagree completely with Helm's edit, and all but one of your newest decisions.

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(used this because personally I believe it was superior to the later depictions of 'fur' which I would dub more as greaser-hair)
take the shoulder for example. This was
definately a step forward, because it made it appear as if it had fur on top of it, rather than just a textured skin. While you can still see clearly defined anatomy, it doesn't look like the muscles are what you're trying to draw attention to. It still doesnt quite look like hair, to me. You also appear to be using your lineart as if it were a solid uneditable substance, which puts a huge limit on the hair texture.

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as an example. if you realize shadows, and how hair acts, it will improve the general texture. Granted, mine could be largely improved with some more appropriate forms, but I didnt allow myself much time for the sake of your challenge, so I only spent a few minutes on it. The outline is one of the biggest issues when considering small resolutions. If you use your outline as anything more than a guideline, you lose proper shading, readability, and style (Outlines make things look cartoonlike, which is bad for fur textures and low resolutions.)