Put the summary of the bug you're having here

What I expected to happen was...:
Horses to stay in the pen until I could gather saddles.

What actually happened was...:
3 of the horses escaped through the exit areas listed below. Saw each one escape so I know the direction.

Steps to Reproduce:
1. Made a 7x7 fence pen for horses (corners 2x high wood)
2. Captured 6x skeleton horses (not. sure if it works on regular) and placed all in pen
3. The horses phased through the wood in the North East corner, the West wall, and the South East corner.

The first Picture is where the horses did get out (Red wool areas) , and the second picture is that of a horse phasing through the wall again, and leaving the pen.