It would be hard to say, but it would certainly would move slower.
It's difficult to determinate speed, it may not variate much if the horse learns how to move in the planet, but expect slower movements as gravity also affect how fast you can move.
We have never take a horse to another planet, so we need to focus on other things we took: ourselves.
When we landed on the moon walking or running is much out of the box, because moon didn't have that much gravity (around 1,6 m/s²) and a simple step will throw us into the air. Earth has 9.8 m/s², and a planet with half of it would have around 4.9m/s². Walking may become awkward, but us are aware of the change of planet, so we will get used to it (meaning we will know that walking normally is not going to work). Horses on the other side will lose it! They wouldn't understand what's going on, they'll become scare because they are used to their weight in earth, and also a different acceleration on a planet means that you will have hard time doing other movements.
Also, a little note, be aware that no living creature on this planet can live in a planet which has such a different gravity, our organism will slowly decay and muscles will atrophy, as our whole body is prepare for living on this planet.