Race - Critiques Wanted

The chest area is, indeed, a bit problematic. I think the main think you're having issue with is that you're wanting to draw it in a profile view rather than a more frontal 3/4 view. A difficult position, to be sure, which is complicated by the fact that you have him rearing up, which tilts his barrel into an even more interesting position. Here's a redline to help illustrate what I'll be saying: [link]

What I've done is blocked out the basic shapes. It's not perfect as it's not from reference, but I think it'll give you a better idea of how it goes together. I traced the shoulders on both sides as well as the forearm (something a lot of people tend to miss when they draw animals) and set the chest in, which in this position will stick out a bit more than the forearms as the forearms are dropped back. I sketched in the knees and then modified the cannons quite a bit. Remember that the cannon is much shorter than the upper arm. Short cannons equal good strong legs as well as a longer, more flowing stride. The ideal is a long, sloping shoulder, shore forearm, long arm, short cannon. I tried to illustrate the planes a bit so you can see the bony front of the cannon and then the brief definition of the tendons that run along the back. Also note that from the front the fetlock joint is wider than the cannon bone. It is also that way in the profile, but the tendons hide this. Hooves are always the most difficult, but remember that they are essentially conical in shape. The left hind hoof that is on the ground should probably be treated from the bird's eye view as I don't think we're looking down on this entire scene, rather probably somewhere level with the horse's chest.

I fussed with his barrel a little bit. Remember that although it is called barrel, it doesn't quite have a barrel shape. It is somewhat bear-like as it narrows behind the elbow and widens before tucking in slightly just before the hip and stifle. I'd probably adjust the position of that left hind a little further, maybe rotating it towards (the horse's) right a little so it sets more under his body than appearing to be out to the side where it stands right now. That 3/4 view is tricky and remember it is a blend of the profile and frontal, not having to choose between the two.

I fussed with the hip and stifle a little bit, but you might want to mess with it further as you decide how exactly you're placing that hip. While horses can suck the hip under them a fair amount, the opposite extreme is not as probable, mostly due to the spineous processes along the top of the spine.

Jumping to the head and neck real quick, looking at my redline, it seems that the head and neck might be a bit on the large side. The way you've set the eye and how thin the neck is offsets this a bit in your original sketch, but it is a bit large for the body side, especially if you're going for an Arab (I saw in the original piece that you mentioned "Black Stallion," who was an Arab, albeit a huge one. And if he is going to be an arab, you might want to refine the face a little more. It doesn't have to be Egyptian-line super-dishy but perhaps just a little bit more concaved would help.

You can see in my redline that I modified the head and neck a little. I think the muzzle might be a bit wide in my sketch, but larger nostrils might be called for, especially if they're supposed to be flared and blowing. I set the eye back a bit on the head (generally the equine head is divided into thirds, the eye is in the first/last third) and also defined the ridge of the opposite eye, which should be visible at this perspective. I also modified the ear position. If you were going for forward, alert ears, this is more where they should be placed, if you wanted them back slightly, remember that the ear rotates and swivels as it moves from one position to the next. When a horse starts to lay an ear back it will rotate out to the side a bit as it moves backwards (as sketch would do a better job of illustrating this, but y'know...).

I know my strong point is horses, but I did fuss with the bird a little. The position of the right wing is a bit odd, especially when compared to how you have the chest positioned, it doesn't quite meet up, which means either the bird has to be moved to fit it, or the wing should be modified a bit to come into better alignment with the body.

Hopefully some of this rabble what useful. (They should give a no rating option for works in progress.) Have fun with this piece, it could turn out quite awesome!