I felt really stupid at first, because my horse always escaped to the top. I also hat a hard time bringing the horses back to center, when they were already way up. But it was very rewarding when I finally managed to get everyone to the finish line! Maybe you give players the ability to adjust the circle range of the mouse by how long they press it? :)
How about adding a leaderboard so people can compare how many horses they got over the finish line?
Very fun game!