Its been so long since I have done photo manipulation, but I can always give some advice.
First of all I have to compliment your cutting on the first picture. The lines on the horses are smooth and natural looking. Good job.
A think I notice with the images first of all is the shadow. Most people forget to put shadows in, so good job on that, but they are off. Always look at your background picture first. As in the first one the sun is low somewhere off to the left and past the mountains as opposed to behind the photographer. You you want the horses shadows to be the same, the should be angling towards the right bottom corner of your picture not behind the horses. It looks like you also used the burn tool for your shadows, I would suggest not doing that as it adds contrast to the colors (making the reds redder...ect) giving it an off look. I would suggest painting a black area then lower the opacity.
For the second image a thing that most pops out at me is that the horse is not blending in. For the image to fit, the horse would have been about 1-1.5 feet deep in the water at least. The horse would be up to it's knees by the look of the background picture...then kicking up a lot of water. Then there isn't that much of a shadow for water pictures, lower the opacity and put it underneath the horse (as it looks like noonish in the background picture), a reflection if you want to add it. Then the cutting needs a little work, it is rough around the edges.
Its a good start! You have the cutting down better than most. If you have any questions feel free to ask. =)