I think this is a lovely picture and the composition is great to my eye - apart from the tree which you mention - but it's quite dark and, even though it's a grey horse, he/she tends to merge into the background a bit and doesn't stand out as much as you might have expected. Would lightning it and sharpening it up a bit get more definition?
But I am no expert and very much a beginner so that is just my very amateur thoughts!
