What I expected to happen was...:
Horse riding uphill 

What actually happened was...:

Horse gets stuck trying to go up a block, once free after riding for a while the horse takes damage or dies (seemingly from fall damage)

Steps to Reproduce:
1. Ride a horse up blocks
2. Wait for it to get stuck on one of them
3. Turn around to break free

4. Keep riding for a while and the horse will take damage after a random amount of time