What I expected to happen was...:
I expected to be able to ride a horse normally. I was able to do this in the immediately preceding beta version of Minecraft.

What actually happened was...:
Pretty frequently, my horse simply stopped moving.  If I did not immediately back the horse up, to "release" it from it's jittery stuck, it would soon begin to be injured very rapidly, as if being injured suffocation. At one point, my horse simply died, within a matter of a second or two after getting stuck.

Steps to Reproduce:
1. Had a horse that existed in a world save from a previous version.
2. Had placed a saddle on the horse.
3. Mounted the horse and began riding it around, after the update to the snapshot version.