Overall I think this is an excellent blend of Equine, Avian, and anthropomorphic anatomy. The arrangement and proportions of the bones are very believable and natural in appearance.
The fusion of vertebrae and enlargement of the sternum are critical for flight and are very wisely included. It does seem that the furcula is absent at the neck, another bone seen in avians but not equines that is an adaptation for flight.
The neck and lumbar vertebrae appear to be perfect reproductions of the equine spine, having 7 vertebrae in each category. You seem to have taken artistic license by having 9 pairs ribs; most mammals will have 13 pairs, and horses have 18 pairs. (I imagien mlp universe equines to have 13 ribs, since they are usually shown with shorter backs than typical equines) The ribs are usually narrower than yours and are closer together. However, if you have done this intentionally and made fewer larger, hollow ribs as an adaptation for flight I do not see a problem
I find it very interesting that you have included a kneecap on the hindlimbs, many people forget about those. In horses those large kneecaps are part of a mechanism which locks the leg into position so a horse can sleep standing up. I'm curious whether you think pegasi would use them for this as well?
I was almost going to point out that the extra digits of the hindlimb were too high up on the leg, but then I realized you had given them their own metatarsal bones. Very nice touch!
The skull appears a good blend between the equine and human influences you are aiming for. I notice the human teeth rather than equine teeth. I don't really have any comments here.
My last thoughts are that the tail seems perhaps a bit shorter than the average equine's tail. My intuition would be that a pegasi would have a longer tail for the purposes of flight control, and maybe thicker bones in the tail to provide the extra strength required for this purpose.
You have obviously put a great amount of thought and study into this. It's the most functional pegasi anatomy I can recall seeing before. Great work!
Thank you very much for the feedback.
In the interest of blending avian and mammalian anatomy, I replaced the fulcra with a sort of modified collar-bone. I wanted something that could serv not only as a support for the wing, but as an anchor for the large neck muscles as well.
The number of ribs was intentional. Most birds only have a few short, abbreviated ribs, and though the chest still needed to look sturdy, I wanted to save wait. I'm glad someone noticed. Both the unicorns and earth ponies will have a more standard number of ribs.
I see no reason not to have the knee cap serve the same function as it does in normal equines. Pegasi may be capable of flight, but they still spend a great deal of time walking.