Hurricane Katrina taught us a great lesson in terms of the value of permanent identification of our horses. Hundreds of horses were displaced, and all but two were returned safely, primarily due to the large number of Louisiana horses that have a microchip (a microscopic chip with unique identification number embedded under the horse's skin; if a horse that has a microchip becomes lost, the rescuing agency will scan the microchip, retrieve the information, and reunite horse and owner).
In Louisiana, a horse must have positive identification to get Coggins papers (showing he's negative for equine infectious anemia), as well as a Certificate of Veterinary Inspection; most have chips.
Ask your veterinarian about chipping your horse. Eventually, all chips and scanners will be compatible. In the meantime, I suggest getting a 125kHz microchip, as there are far more scanners for that chip type than any other, increasing the chance of recovering your horse.