When she was in pasture the pony was her herd and her security. When she's now left alone in the barn she just knows a predator will get her because her lookout isn't there. She is trapped in that cage you've put her in. There is a way to deal with this that requires time and infinite patience. Leave the pony and goat in and take her for a walk. When you feel her tensing, and before she begins to react, walk her back so she can see the pony then back again. Each time you leave the goal is to get a little farther, even if only a few steps. This may take you an hour. Your are playing games with her stress levels, a yo yo effect. As she gets farther away it goes way up, as you return, way down, up down, up down. She can endure only so much of that then will go with you. Now, if time permits, do this again the next day only before you start set a pan with some treats in it farther than you'd gone the previous day. Now, when you get there she gets a wonderful surprise. On the third day set the pan even farther, way farther. Her focus will be more on finding the pan than the pony.