I do believe that all of these have definite potential and any one of them would make a lovely portfolio piece if followed to completion. That said I'm going to work clock-wise starting in the upper left:
Watch out for tangents, unless you're doing it intentionally. There are two with the unicorn on the left formed with the moon. I would suggest either moving it over so it is fully within the moon's frame or going the opposite way so it is purposefully breaking the frame rather than flirting with the edge. His wither appears a bit high up and forward compared to how his shoulder suggests it should be. The horse facing us more is nice and dynamic in its movements, though there is some discrepancy between the angle of the chest and the angle of the forelimbs. The right foreleg is more in profile than the chest would suggest, especially in the fetlock, pastern, and hoof area. I know this area gives me undeniable difficulty, especially at odd angles. The left fore looks pretty good until we get down to the hoof, which looks straight-on when it should be turned to its right for a 3/4 view (unless it's toed-out, but most artists try to draw near-perfect conformation and shy away from such things). The body is nicely put together and the hip to stifle seems pretty solid. There is some weakness in the foreshortening of the lower hind leg (though it can be hard to tel at this size). The right hind leg doesn't seem to be set quite where it should be, either angled back too far from where it is positioned or slightly misplaced, even if it is tucking up and to the side as you see some showjumpers do to avoid a rail.
I like the blue and green color scheme and would be interested to see where you go with it.
The next piece is full of energy in its subject and looks like it could be quite a fun piece to work on. Be careful as you build up the hose's shoulder as again, the wither seems a bit further forward. I'm not sure if he (she?) is hitting the top of the dune or if the feet are lifting from the top of the dune. The former seems more likely, but some of the strokes suggests the latter, which is odd considering the horse is behind it. The composition could get a bit tricky in this piece as you have a very strong line of action pushing the viewer to the upper left of the piece through both the dune and the horse's body. Currently the only thing drawing us back is the downward tilt of the horse's head, which might not be strong enough to bring us back around so be careful with that one.
A note for overall equine anatomy (and one tends to see this a lot, actually so you're not alone): Ideally the forearm is longer than the cannon on the front legs. This provides a more efficient stride as well as being more structurally sound. Not to say that no horses have long cannons compared to their forearms, but the ideal is for a longer forearm and a shorter cannon. A most excellent book to bring you beyond simply being anatomically correct, but also conformationally sound is "The Horse Conformation Handbook" by Heather Smith Thomas.
Continuing on to the next piece. I love the action and dynamics of this image. I don't think I have much to say about this one other than keeping tabs on where your shadows are coming from and pushing the shadows in the background and you bring it to a more fully realized state.
The Akhal-Teke is a handsome fellow. I'm not as familiar with the breed, though he seems a little thick and upright through the neck. The right hind leg should be a little more open through the hock considering the angle of the stifle (the hock and stifle joints are actually connected and a horse cannot bend one without bending the other, same for extentions) or a little more closed in the stifle compared to the angle of the hock. I do realize that these are nit-picks and overall, as I said at first, he is quite the handsome fellow.
I really like how this still life is going. The warm brown background suits it, though I'm sure you know that it needs a little something more than a flat-colored back drop. The color scheme appears nicely unified, though I would watch the tangent that appears with the petal that at this size appears to be just touching the frame rather than definitely breaking past it.
In the final piece parts of me wonders if it wouldn't work better if the girl were paying attention to her teddy bear rather than the viewer. Having her stare at us, to me at least, stops the viewer from going deeper into the scene. She's staring at us as if we've intruded and it is hard to get past her face. If she were turned towards her stuffed bear I think it would be easier for the viewer to feel he/she has "permission" to dive deeper into the world that he has stumbled upon.
Hopefully my rambling helps you as you continue to work on these pieces. If you have specific questions about anything I mentioned feel free to ask.
Thank you thank you thank you for writing such an AWESOME critique.
Moon and Water horses: I see what you mean about the tension with the left unicorn and the edge of the moon. I will move the unicorn outside of the moon's edge so that he is bursting into the darkness. It will add activity to the dark sky and liven up the composition. I haven't compiled my array of references for this image yet, so the anatomy is still really roughed in. All of your points on the anatomy are valid.
Dune horse: The horse's hooves are hitting the top of the dune and causing the sand to spray. It needs more refinement and I need to do some more research on how sand behaves. I'll see what I can do to get the lines of movement to draw the viewer back in. Any suggestions given what i have to work with?
I always want to make that mistake, I like my horses leggy, there are only a handful of breeds that break that rule (Akhal-Tekes being one of them) and I tend to want to draw all my horses with long cannon bones.
The Akhal-Teke: Akhal-Tekes are very upright in the neck, their heads are above the rider's hand even when held in a relaxed state. He may be a little to thick and archy in the neck though, I have to gather better refs. I'm going off of several references for the pose, as not one picture matched what I had in mind, so something was bound to get screwed up in the anatomy.
The Iris: That petal is just touching the frame, I don't want it to break out of the frame, so I will add some space to that edge. There is going to be more of a background, and even the flowers don't their dark values in yet.
The Girl: I want her to be looking at the viewer, as if we have walked in on her playing so near to the edge and she has paused to look at "us". Although I do want the viewer to feel comfortable looking past her and at the ruins, so I may shadow her face more so that it isn't so bold. The thing I am struggling most is with the ruined buildings. I am not good at architecture and rubble is even worse. I feel like the buildings closest to the girl are not big enough, even though she is up high in a building, on a sort of "blown out" balcony. And the atmospheric perspective/fading is giving me fits too.
You are most welcome!
Dune horse: I'm not sure if you were set on a completely clear night sky but maybe some slim clouds heading back towards the moon might help to bring the eye around. The shift in darkness does this a little already, but in the finished piece I imagine that line will become much more subtle. Speaking of which, be careful as that line currently arches up around the horse rather than settling independently in the background, unless you want that interaction. I'm also debating if something should be done in the lower right, something subtle and suggestive as you still want a place for the eye to rest from all the action in the upper left. A dune in the background rising up slightly to the right perhaps would be quiet, but gently direct the viewer up again.
Akhal-Teke: He is in a bit of a "look at me" pose, which would cause him to elevate his head and neck as well as arching and generally rounding his frame, as can be seen by his nice step underneath his body. Usually the muscles on the underside of the neck are also softened in this position as the base of the neck raises and is supported by the topline neck musculature rather than the base dropping and braced with the underside (you can see the forehand is slightly elevated despite his foot still being on the ground). Of course on a horse normally ridden in an inverted frame the underside muscles can get quite overdeveloped and still be visible even in a relaxed state. This is probably more information than needed and pardon the ramble...
Again, not super familiar with the breed, but the images I have seen have always been very lithe and slender, especially through the neck and barrel. He has a bit of a thick "Baroque" neck with a rather thick throatlatch and cresty neck, which I think you'll pick up on as you do more research. I wouldn't say his anatomy was screwed up, just needs a little tweaking to pin his breed down.
I know artistically long cannons seem "right" a lot of the time and many artists tend to lean that way. I think it's something you have to train your eye to just as with anything else. Of course once you're aware of it it becomes easier to correct!
The flowers should be lovely once they're finished, keep up the good work.
The Girl: I think the closest building should probably break the upper drawing plane to help bring it closer as the background is a bit set back, which probably contributes to the feeling of not being able to get past the girl because she still seems a bit apart from the background and isn't quite set in just yet. Almost like she's on the ledge of a rocky ridge that's fairly removed from the city below. If it is to be a balcony I would suggest having some sort of remnant of a railing (even if it's just a nub or two) and include some of that greyish color of the buildings. It can be hard to tell what all is there at this size so pardon if I'm off on some of my assumptions. Yeah, the building on the left should definitely be larger and perhaps the one on the right as well. At the moment both of them are of similar size as the ones that are faded further into the background, which makes for a rather shallow drawing plane. I think as you get the building sizes a little more staggered and less homogeneous the atmospheric perspective will settle into place better.