Just initial thoughts, the horse seems almost sort of dull. Technically yes the horse is in shadow but it's almost hard to see. I would recommend upping the contrast and brightness of the background and adjust the horse accordingly. You could also darken the foreground (Multiple layer, large dark green airbrush) to draw more attention to the horse. Part of the dullness I think is also the lack of color on the horse's body aside from the yellow highlights. Don't be afraid to add shadows using dark green/brown even though the horse's base coat is black/grey. Almost all black fur has either a dark blue or dark red (brown) undertone which would help make the horse look more natural.

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I absolutely love the texture and transluscency of the fabric!! I would just say to be careful about stark white outlines, particularly around the underside of the horse. I understand that you're trying to make the horse stand out a bit more but it just feels unnatural. To achieve a similar effect, I would add a new layer behind the horse, set it to Hard Light, and use an airbursh to create a soft gold outline around the lit parts of the horse, adjust layer transparency and blurring as needed.

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The hair is completely gorgeous. I just have a lil question about lighting. Looking at the section of hair in between the horse's back legs, it is highlighted once it emerges from behind the farther back leg. However, this highlight is not carried over to the horse's leg or underbelly. I think it's up to you if that area is highlighted or not but consistency is important ;D

Last thing about lighting is that the horse's face has some very strong highlights despite being on the complete opposite side of the light source. Overall, I love the definition of the musculature and the piece as a whole is very very lovely <3