I'd just go get my horse. If she threatened to charge me with anything I'd quickly let her know murder may need to be included if she didnt get the heck outta my way. Tell her that you will see her in court but untl she wins a court case that it is YOUR horse and will remain in your posession until a judge tells you otherwise.
As somebody mentioned, I would contact the people who had leased the horse as a companion. They quite possibly don't know what is going on, or else may not be as good of friends with this person as you think. Furthermore, you don't know until you ask. Did this person walk all over you in your time with her? Is she just a downright bully? It sounds like she knows your not going to stand up for yourself and is taking what she can. You need to stand up to her now and let her know you will not be bulled.
I'm glad you are keeping a papertrail on everything... and I do hope that everything works out for you.