What a stunning piece! I love the monochromatic palette and I think in general it is close to perfect already.

But you did ask for some critique, so I'm obliged to offer some;

Composition - This image works a lot with space. Closeness; the big dark equine in front, the middle where the river is gently curving away, and the far away bird, signaling space and direction.
To enhance the sense of space, I would increase the canvas size a bit sideways. Just adding a 1 or 2 cm to the left side, and 3-5 cm what you add there to the right side. This will enhance the vastnes of the image.
The bird shape could be moved a bit to the right as a result.
As the landscape fades out into a pale grey/white, the bird will stand out more and at the same time balance the 'weight' of the image out.

Technique - The landscape is beautifully painted, I like how you mixed a painter and sketchy style together beautifully, it blends just right.
That beeing sad, the horse looks just about perfect, matching the style of the rest of the painting. But the fetlocks and the tail lack the same care and attention you put into the mane. Use the mane as your guide - it is thick at the roots but lighter and more flimsy at the edges. The tail looks too thick and a little akward. There is a part where it flows over the leg where it looks perfect though.

I think that about sums it up! It looks absolutely gorgeous, and it's just those finnishing touches that will turn it into an absolutely magnificent piece!