I do not think this is her trying to get one over on you. Not at all. I think this is fear. Sometimes horses will act aggressive but really they are scared to death. Have you ever seen a horse out on the pasture with several others and this one particular horse freaks out on all the others? While the alpha horse is very calm and direct and doesn't throw a fit when he tells the others what to do? This is insecurity. So to me, it sounds like her insecurity is coming across as aggressive behavior. If I was in your situation, here is what I would do.
Go out to the pen where she is at, take a chair and sit on the outside of the pen. I would not go in if she is still acting unpredictable, just to be safe. Sit there for 30 minutes while reading a book or something and observe what her behavior is. Does she come and investigate? Does she ignore you? Does she start pacing? Etc. The reason I would do this is because this is going to get her curiosity up. She might not have really ever been given the chance to see a human in this state before. If she comes over to you and wants to touch you, as long as she isn't trying to bite, etc. let her but do NOT touch her first. She has to be the one who initiates the touch. When the time is up simply get up and leave as if she is not even there. I would do this exercise everytime you go out until this issue is resolved.
After you have been away for a few minutes go out to her pen, but now you want to see if you can get her. Here is where it's going to require a lot of patience on your part. As you enter the pen take notice of her ears. If her ears are full forward on you, you have permission to approach. If at any time she pulls one ear away from you, turns her head away or she pins her ears STOP in your tracks and wait. This is her telling you that you do not have permission to come into her space. We want other humans to be sensitive to our personal space, and horses are no different. If you just barge in there you are showing her you have no sensitivity to her personal space and thresholds. So, say you get 2 steps in and she pins her ears. Stop and wait until she brings them more forward. This could take awhile but you must prove to her that you understand. If you are waiting and waiting and waiting take a couple steps back and turn side-on to her. But be patient. If she turns her butt to you throw your halter and lead at her butt, not in a "You rotten horse don't you dare do that!" kind of way but enough to get her to face you. At this point if you make this into a smacking fest and get aggressive she will only fight back. And you will not win. If she comes at you at all in an aggressive way drive her away like an alpha horse would do. Focus on driving her front end away. The more we can drive a horse's front end away the more we are instilling in them that we are alpha. If she comes toward you in a nice way back up as if you are drawing her to you. Let her make the first touch, find a good itchy spot, rub her with the halter and slip it on.
Say you get the halter on and all is good. Take her out to eat some grass and maybe go for a nice walk on-line. You need to invest in the relationship again so she trusts you. I would also put her in a good quality rope halter so you can teach her to give to pressure from her poll. When you do this exercise, put a tiny ounce of pressure on the lead and SLOWLY close each individual finger on the lead. This will add pressure to her poll and if she goes to pull away hang in there with her but don't try to force her. Go with her so that she doesn't feel trapped. When she makes even a TINY move in the right direction release all the pressure and give her plenty of time to think about it. Also teach her to give to pressure on other places of her body. Lead her by the leg, ask her to move her front end and hind end away from you by SLOWLY adding STEADY pressure (not poking) until she moves over. When she does stop, rub her.
. Pretty boy though and neat color. Is he a liver chestnut or faded black? 
