Yes all horses have to be micro-chipped for registration to most societies these days. However, this is still not fail-sail as thieves can actually remove this chip and replace it with a different one. The only real deterrent is to brand them with a freezemark, basically a very cold iron is applied to the skin, on the shoulder or under the saddle area, for a minute and the hair will grow back white. It's painless for the horse and permanent and because it's so easily identifiable, it's been proven to be the best form of anti-theft protection.