This is a sticky situation. Honestly. You had a 6 month lease with her, it wasn't renewed. There wasn't much communication. You willingly took care of the horse while the owner did not contact you about it. I would talk to a lawyer about this and see where you legally stand. Is she willing to sell the horse still?
I have a question maybe some of you know the question when you have a verbal agreement for a free lease why do you sign a bill of sale and give the leasee your papers to your horse. Legally how do you know your going to get your horse back?
We had a similar case in my state where the leaser sold the horse without permission. The person who bought the horse had to give the horse to the person who originally leased the horse out. Trust me unless she emails you and says you can have the horse don't assume it's yours or your in for a big lawsuit for theft.
You've had the use of the horse for longer than the agreed upon time which has benefited you. If she wants her money, pay it. Perhaps this could wind up in court and perhaps you'd get out of paying for the horse but one day it will come back to bite you as disreputable people to deal with.