I want my horse back?

You can speak to her to see if she would be in agreement with you taking the horse back. However, if she wants to keep him, you have no right to take him. It doesn't matter if there was a witness or if there was an exchange of a dollar. You made this agreement with this person and she has fulfilled her side of the arrangement. If she wants to keep him, she can and you should do the honorable thing and keep your word. It is not about the knit-picking so that you can have what you want right now. You made an agreement and you should honor your word. You should do the right thing.