This rare white horse is found in the area around Lake Isabella in the snowy mountains northwest of the map, near the hunting grounds for the Legendary White Bison.
It blends really well with the landscape, making it really hard to spot. It’s even harder to spot it if the weather is bad. When that happens, you’ll have to camp it out until you get a clear sky. It seems to appear only during the day. Use your binoculars to scan the surroundings as well for a better chance in spotting it pre-emptively, and even better, tracking it.
Once you found it, you may have to approach it carefully. It’s better to approach it with the Cover Scent Lotion applied to avoid spooking it prematurely. Once you’re in range, throw your lasso to catch it. You’ll still have to calm the horse down and ride it and break it. (You can refer to this guide’s Horsebreaking article for more info). Once you calmed it down, this wonderful, unique horse is yours! Make sure to deliver it to a stable (the nearest one from here is probably the one in Valentine)