It's not that difficult, depending on your artistic skills to start with.
Really it's mostly about erasing the background on the horse (erasor works best here, actually..you can try the extract filter too, or use a layer mask..but erasing with the erasor is good enough). Smudging or blurrying some chosen parts, like hooves and mane, can be good.
Then you chose a background, put it on a different layer.
Then use the layer adjustments (especially hue/saturation) to get it to match. Perhaps use dodge or burn to create shadows on the background from the horse.
Add text if you want, and just mess around untill you're happy.
It's not difficult to make it, technically, the hard part is yo get a good feeling in it and not all the tutorials in the world can fix that.
Sorry Horseluver50, but it doesn't have to take 5 years..:P It can take a month, it can take 10 years, all depends on skill, talent, how much you work with the program and how quickly you learn all the things there is to use :P
I've had photoshop for a year or so before making these..and I'm not usually into photomanips. I've only made a handful of them :) as one Crow unicorn Creepy horse Running Crow
What really helps tho is a graphics tablet, so that you have a pen to draw with.
And the more you have to paint to make it mend, the more diffiult it ios. There are lots of people way more skilled than me, but then again..I don't do manips much :)
I'd say go for the tutor offer, it's great to have help at hand when there is something you know is do-able but don't know how.