Let's see...
Well, you notice how in your ref, the tail isn't sticking up? You can still see it, but it isn't straight up. Also, because your tail is separated like that, it looks really abnormal, because in real life, the tail wouldn't stick up like that and wouldn't be separated into pieces.
And you could move the front legs up a little bit. See, in the ref, it looks like the back legs are closer to the ground than the front legs. If you did that, it'd make it look more like it was jumping. Because now, it looks like it's a standing horse with a whacked out tail.

And the mane is just a tad bit long, and because of it's color, it makes the neck look really thick. I'm not saying that you change the color, just make the mane a bit shorter.