Wow, that is absolutely crazy! I deffinatly feel for you! But like others have said, keep a rather calm head. You have every right to be furious, but you cannot let things get out of hand! My advice to you is to continue trying to file a law suit against her, although in the process make SURE you have good proof. Because if this lady is crazy enough to try and STEAL your horse, then she is crazy enough to come up with a good enough lie that can get her out of court.
I suggest taking pictures of EVERYTHING! Such as the padlock on the stall, and your horse locked inside of the stall.
ALSO make copies of any receipts and documents stating you have paid every board fee required from her , proving you owe her no 10,000 dollars!
But like we have all said just keep calm :) it will work out fine I promise!
P.S. If worse comes to worse just buy some industrial lock cutters, run out there at midnight, and sneak you horse away!