In the latest snapshots you can't ride or tame any horses. At least I can't. I will add some more videos showing it soon..
Horse taming bug in action: https://gfycat.com/RightSentimentalGermanpinscher
No matter how long you try, the horse can't be tamed. Even with newly spawned in ones.
Earlier tamed ones will not move once you ride them, they will move out of themselves trough.