Its hard to explain what happens but I know that it is a bug.

What actually happened was...:
The horses were standing in their pen at the stables and five to ten minutes later they disappeared. I logged off and back on and they were back in their pen

Steps to Reproduce:
1.Stand close to where you put the horse in a pen
2. After placed in move away from where it is
3.Check back about 3-4mins and the horse disappears
4.Log off and then log back on
5.The horse is back in the pen