Thanks Loosie for the suggestions - I had seen the "Tie Ring" but couldn't remember what it was called to order one. At this point, I'm getting incredibly frustrated at the situation. The barn owners are coming out tomorrow to try and help us figure out how to get mine back in the smaller field.
By catching the mare and bringing her down, when the filly wasn't staying super close, we accidentally made the mare weary of being caught. We can approach her, and she'll actually come up to us too, but at the first sight of a lead rope, she turns and leaves.
Hopefully tomorrow we can figure out how to get them down - possibly corralling the entire herd into the smaller pen and releasing them one at a time, but that's something the barn owners need to be present for.
To make matters worse, the filly lost her halter. It was a break-away/safety halter, so I'm assuming that it worked as it was supposed to, but we couldn't find it anywhere in the field to try and figure out what she got it caught on in the first place.
Trying not to get discouraged, but I'm pretty frustrated at the moment...fingers crossed we can gather them up again tomorrow.