Rivendell Steeds said:

I'd say you have a lot to work on, but to make your art worth enough to be sold decently, you need to be able to do color changes, define the muscles properly (and smoothly), ground your horse into the background, and have definition to your hair. You can find many useful tutorials in this section of the forums, a few guides in the member help section, and more in places like Deviantart, Youtube, etc. Many artists are also willing to offer tips on how to accomplish certain aspects if you ask!