Repossess her. I used to repossess dogs when I did rescue if the person violated the terms of the contract. Take a copy of the signed contract with you and pick up the horse. If the cops show up, you have a signed contract showing she is to be returned. I'd also take a copy of the CL ad. Good Luck!
Hmm, in future situations like this (if you wanted to make sure that you could get the horse back if the people didn't want it any more) wouldn't it be much more easily enforced legally by giving the horse on a free care lease with the agreement that this person could keep the horse as long as he/she cared to with the understanding that if the horse's condition started to suffer or she no longer wanted the horse, it would have to be returned to its owner.
If the transfer of actual ownership never takes place, then it isn't a problem to go get YOUR horse.