I would say perhaps a hit and run poster, but if not hoped they worked it out...all I know is if someone didn't want my horse I would want it back asap instead of wondering what they were going to do with it.
I think it all depends on how you feel about the whole situation. If you are dead set on keeping the two you currently have, get any and all written conversations more specfically those pertaining to a conversation about the actual trade and any agreement that came with it. Also if you are that worried she might randomally show up, contact you local police department now and explain the situation giving them the heads up.
But if you are not dead set on the two you have I would consider a trade back. If I were in your position I would be worried about the potential out come of the older horse. If they feel they cannot deal with it they may get rid of it by whatever means they can (i don't think I need to list them).
Your best bet is to contact the lady and have her explain EXACTLY what happened and explain your side about how the horse has never bucked in the past etc etc. You should maybe consider investing some time in a trip to visit this lady and ride the horse yourself either to see what they are doing wrong or to see for yourself.