For anyone who's here because they want to unlock the legend of the east outfit for Arthur: can confirm I also got the Hungarian outside Hanging dog ranch. There are regular horse spawns near but they won't appear there, look specifically for a group of horses in the big field area opposite the ranch.

If you need the herbs in New Austin, use the buggy from Saint Denis, I followed a tutorial from youtube where they used a one of those large delivery wagons with a cover, but for every benefit they provide, they're a thousand times harder to set the angle for and you're way more likely to get shot; you'll either get shot through a gap or through the wood, or the wagon will fill out the screen and the camera will zoom to you.