This.
I am far from a legal professional, but I absolutely do not suggest “stealing” the horse back in the middle of the night. This is a bad idea for so many reasons. At best, it could look really bad in civil court and could result in YOU having to pay HER for breach of contract (for the 6 months notice). At worst, it could result in criminal charges or even possibly being shot or something as an intruder on her property. As great as it may sound while we’re all chatting on this forum in a safe environment, this is a bad, bad idea.
I think she’s just bluffing to try to get you to back off. Does she have proof that she has tried to contact you for board or anything like that? No. Do you have proof that you signed a free lease contract? It sounds like it. As others mentioned, gather up any and all evidence that you might have, and be prepared to fight.
And this is a reminder for all of us…you should always, always get important contracts in writing and have them notarized. It can seem so awkward when you’re dealing with family or friends, but things can turn nasty later, and it’s just better for everyone.