I've had a single player world since release version 1.7.x perhaps even earlier. I'm now in the most recent update 1.11.x But I've noticed for the last few updates. An issue with tamed horses. It's more apparent in the nether, as I have dug tunnels in the ceiling. After seemingly random periods of time. While riding a tame horse, mule, or donkey. They will stop and buck much like an untamed horse. I've noticed even newly generated horses, from newly generated chunks, seem to do this eventually. It is quite jarring, considering that horses are faster and less noisy than mine-carts. I have edited the world generation a few times. The world gen was initially set to amplified, to allow for islands in a massive ocean. However, I'm not sure if this could influence mob behavior.