Haha,
Welcome!!
What a great piece.
Great colors and sense for volume.
I really like the exaggerated pose, quite funny.
I would just try to work on focus, on readability.
The tail has a lot of detail
lots of sharp lines and contrast, so do the horse legs compared to the Face and horse head.
This way it catches a lot of attention while the real emotional parts, like face and horses eye are left quite undefined.
Lead the viewer directly to the ambitious racers face and give a clear expression there (I guess I see a very concentrated eye behind the glasses, and a tongue)
After, lead the viewer to the clear horse expression, which has mostly wide opened eyes while racing, a bit panic like.
This way, you would achieve a nice emotional contrast in this part of the imagine.
Maybe a bit sharper detail on helmet as well.
On the other hand, I would take away strong dark lines and attention from tail.
Ahh, well and some shadow for the arm
Just my two cents