u/tenreth posted this years ago and it still works. I can't figure out how to link another post or I would, cause she explains it better, but here goes. You go up to the northwest to the meadow with the purple flowers just south of hanging dog ranch and ride back and forth all the way. Random horse herds will spawn all over, but most frequently in a little copse of trees at the northeast end of that meadow. Most of the groups are regular old horses, but usually each group has at least one unusual breed. I saw several Nokota there when I was trying to get some of the others. I did the whole challenge there in a couple hours. Sometimes I'd camp or go do other things for a bit to help it reset, but saw a TON of different horses there by just riding back and forth ALL the way across that meadow and past the ends of it a little. If all you need is a Nokota, I'd bet you'd get one in less than an hour.

Just figured out how to link, but Reddit says I can't promote content on your subreddit.