What I expected to happen was...:

Horses can swim you know !

What actually happened was...:

Everytime when i traveling on my horse (no matter what type it is) and jump down in the water, horse is just disappear ! I just hear a sound of a horse. BUT when i went far away from a bug (render distance 8 chunks) my horse just appearing swimming in the water (on the buttom of water surface)

Steps to Reproduce:

1. Ride a horse.
2. Jump in the water. (deeper than 2 blocks)
3. Just wait until the horse goes down or swim forward.

(i)You supposed to jump off (automatically) and horse will disappear !