While several people on this thread have suggested contacting the local police, the police consider this a civil, not criminal issue. In essence, JB is in breach of contract with you, which is different than if she literally stole your horse off your property. You entered into a sales agreement with her, and she didn’t make payments as stipulated in the contract. While this is tragic, it’s no different legally than someone who goes to the local car dealership and buys a car signing a contract to finance the car over time. If the buyer doesn’t fulfill the contract, the police don’t arrest the buyer. The car gets repossessed.
Since JB has evidently posted your filly for sale on the internet, make sure you print the ads or screen shots of your filly before asking the websites to delete them. They will come in handy as evidence if you go to court. You mentioned that you have contacted an attorney, which is good, because you will probably have to sue her for the return of your filly.
As for locating the filly, ask a friend to call JB about the ad and express an interest in buying the horse. If JB is planning on selling her, she will have to let prospective buyers look at her.
Please keep us updated on the return of your filly!