Remember the last season episode where right before our crew of intrepid survivors walks into the setup at Terminus Rick is showing Carl how to make a snare to trap game? He explains how the animal is funneled into the trap and is caught before it knows its even in jeopardy. Remember that?
So look at the metaphor in tonights show. The horse has been running around during this crisis and has been surviving just fine. And then…someone tries to reintroduce it to civilization, and once its in the pen behind the fence it gets swarmed and killed by zombies. It was safer outside the confines of the pen, taking its chances with everyone else.