This all seems to be a bit up the wall.
Unless things have changed a great deal, which, with the extra airport security, I very much doubt, it is all seems impossible to have occurred.
For a starters no minor would be allowed to travel with their horse, this has to be done by an experienced groom. The child would be travelling as a regular passenger.
You cannot just 'get on a wrong plane' with a hire. It is hard enough to do just as a passenger, correct boarding passes have to be shown.
The horse would have had to be in airport stabling and crated to be loaded, they are hardly likely to load onto the wrong plane.
Even if it did happen it would be easy to trace where the horse was sent to as this animal would have to be unloaded at one point or another.
The fact that the OP is not on good terms with anyone seems strange because in situations like this people forget their differences and bond together in a common search.