Microchipping is becoming prevalent, if not required throughout the horse world. It is not for locating missing horses. Rather, competitive organizations and breed registries are now requiring it for ease of identifying individual horses.
Source: The Horse, January 10, 2017. (tiered access).
The entire process, including the price of the microchip, usually costs less than $100. Upon inserting the chip, the veterinarian scans it again to ensure it is transmitting readable information. Then the horse owner registers the chip with the appropriate microchip company, as well as the desired sport organization and/or breed registry. This is a one-time process that needs no repeat fee or renewal.