Tiggr - I have only questions at this time. When you went to board with her, did you have a written contract of any sort (particularly, a contract that talked about the training aspect)? If you allegedly owe her money, did she ever request it of you (again, particularly in writing)? If she kept the one horse since you allegedly owe her $10k, does that horse have a value of $10k that has been documented? If it is not valued at $10k did she give you any explanation (once again, preferably in writing) as to why she wouldn't keep both horses to try and recoup her alleged losses? Since you said you filed a suit today, you probably should not answer these questions at this time as it may be used against you somehow (I'm certainly no legal person). My purpose in asking these questions is a way of trying to see how it might look from the court's logic in determining the facts and where the strength is in your case.
Stay calm and keep thinking. Best of luck.