I definitely think there's a good deal of growth here, even over the course of these few drawings. Your last one - the fox - shows the strongest construction overall, although I think all of your drawings rely more on memory than direct observation than they should (to varying degrees). Basically when we get really focused on construction, we tend to forget to look more frequently at our references, and that can result in some elements of proportion and pose to be based more on what we remember seeing, rather than what is actually there. In turn, we get things being a little off, a little oversimplified in places, etc. So while it's important to think about how we're constructing a form, how it sits in space and how it relates to the other forms around it - we need to make sure we're observing our reference closely and frequently to ensure the form captures the element it is meant to as accurately as we can manage.

One issue I felt really stood out in your bison was that when you added additional forms, it's clear that you were trying to make those forms wrap around the existing structure, but it didn't turn out that well. I point that out here. The way in which your lines curve will either enforce the idea that it's wrapping around a given form, or completely contradict it. Avoid straight lines, as well, when you want to create a sense of wrapping-around.

It's also worth mentioning that there is a lot of nuanced forms present along a structure, and I feel like you're ignoring a lot of them. If we look at this image of a bison, you'll see how I've traced over a number of additional forms throughout the body that all integrate with one another, creating a great deal of subtle bumps. Your constructions can definitely be taken much further without getting into detail/texture than you have been thus far.

That said, getting back to your fox, I think it's clear that you are pushing those further with the fox's legs - though much of the body is still fairly bare (and I think its torso was drawn to be pretty thick and broad, whereas foxes are usually much more sleek. The head construction, which I mentioned before, was definitely fantastic.

One thing about the fox though - I love that mass you drew along its neck, it wraps around quite nicely. Do be mindful of avoiding having those masses cross the center-line of the body though, as shown here. Since the bodies are generally going to be more or less symmetrical, having that form overextend so far ends up feeling a little odd.

All in all, I think you're showing a great deal of progress, but that I would like to see you push it a little further with two additional drawings. If these come out as well as the fox did, I'd be perfectly pleased, although I'd be thrilled if you explored how far those additional forms can be pushed.