I really, honestly think you should just go get him. I've heard of his happening before and people literally cut the lock, leave a new one, and ride their horses to an awaiting trailer.
They then settle the money aspect in court.
No one can make you do that though, and it seems like you're a bit too timid to try it. It'll sort itself out. You may be able to bring the people who were using your horse as a companion animal to court to testify that they had him.
If you owed her 10 grand and she's keeping a horse from you because of it, she would have had to keep the other horse as well. That was a stupid move on her part (much like the rest of her actions).
While you're at it, make sure you toss in "pain and suffering" as well as make er pay the lawyer and court fees when you win.
And those who were seen dancing were thought to be insane by those who could not hear the music