When we think about horses being stolen, we tend to imagine a trailer pulling up to the barn in the dead of night, masked strangers, an immediate all points bulletin the next morning.
But many, maybe most, cases these days are more like this: disputed ownership. The fact that the police dismissed this as a civil case at the start means that there was reasonable doubt as to who the horses really belonged to, and the OP could not prove her own ownership enough to make this a criminal charge.
I recently watched as a similar dispute erupted, unfortunately, between two sets of people that I’m friends with. Two horses were sent by owner in financial distress to a lesson barn, with no contract, bill of sale, or official change of ownership. It was unclear to me whether the horses were there to be sold, on a free lease, just given away, or abandoned. Owner pays nothing for horse care for two or three years, then decides to take horses back at a moment’s notice, because they have a potential buyer for them. Lesson barn refuses, says they now own the horses; arguments erupt; owners take back horses in middle of night; lesson barn wants to say horses were stolen, put a lien on the horses, get a lawyer, but ultimately nothing they can really do as there was no legal agreement, and the basic cost of retaining a lawyer is going to be more than the horses were worth. Messy all around.
This is a different situation and different outcome than the OP describes. But things get vague and messy if horses are abandoned with their caretakers. There is some room for caretakers to claim that a horse was abandoned, or that they needed to sell it to recoup expenses. And things get even messier if one party launches a court action, but isn’t able to follow through on it.
If I were the judge I’d want to hear the whole story from both parties, and from a lot of other witnesses as well. But I wonder if the whole appeal to law is even worth it, after all this time, if the horses have disappeared off the face of the earth?