All the Horse Breeds in Red Dead Redemption 2 and Where to Find Them
For the Horseman 10 Challenge in Red Dead Redemption 2 you must find all Wild Horse Breed Locations and break the horses by catching and riding them. There are 9 Wild Horse Breeds in RDR2 that are required for this challenge, the below guide shows their locations (and some additional ones which are not needed for the challenge).
To break a wild horse you must approach it slowly while aiming at it with 
/ 
and pressing 
/ 
to calm the horse (when you're close enough to it, this prompt will show up in the bottom right of screen). Keep approaching slowly, always calm the horse when the prompt shows up until you're so close that you can mount it. The wild horse will try to throw you off but you must stay on it to "break" it. To do this, hold
and move 
in the opposite direction of the horse's movement to stay balanced.
The easiest way to spot the horses is with your binoculars (can be selected from item wheel). Look for horses through the binoculars from a distance. When you see one, hold 
/ 
to study it, thus revealing what type of horse it is. Then you know if it's the one you need. They all look very similar and even within a breed they can have totally different colors so it's hard to tell if you don't study the horses.
The Arabian is not required to complete the challenge, only the horses listed below are needed. They only count after having done the first 9 Horse Challenges.
Note: the below locations are just the most likely spawn points where the horses are confirmed to spawn for sure. They can spawn within a certain radius, see the red circles. However, just like any other animal, their spawns are random so they might not show up on every visit. If there's no horse for you in the red circle: save the game, exit, reload. This refreshes the animals in the area. The breeds that can possibly spawn often overlap (there are usually at least 2 breeds spawning in the same area). These are not the only spawn zones in the game, wild horses can show up in lots of other places, but the below locations are the best to find the specific breeds.
Wild American Standardbred
In the very north of West Elizabeth region, at Little Creek River. The Wild Morgan can also spawn here.
Wild Morgan
Spawns in the exact same area as the Wild American Standardbred.
Wild Mustang
In New Hanover Region, in the "Heartlands" sub-region. Wild Tennessee Walker can spawn nearby.
Wild Tennessee Walker
In New Hanover Region, in the "Heartlands" sub-region. Wild Mustang can also spawn here.
Wild Appaloosa
In Ambarino Region, just south of Wapiti Indian Reservation and near Donner Falls.
Wild Hungarian Halfbred
In the area around Blackwater you can find the Hungarian Halfbred, one of the rarer breeds in the game.
Wild Kentucky Saddler
In the area south-west of Blackwater.
Wild Nokota
In New Austin region, south of Armadillo. The same area has a high chance to spawn the Morgan breed.
Wild American Paint
The last breed can be found in New Austin region, south of Tumbleweed.
Bonus: Arabian Breed
The Arabian isn't needed for the completion of the challenge but it's actually the best wild horse in the game. It's a unique horse (can also be bought randomly from stables in Blackwater, but it's not always available there).
Bonus: Unique Tiger-Striped Mustang
This unique Mustang can only be tamed after reaching the epilogue. It's a one-of-a-kind horse like the Arabian. It's found in the south of New Austin region, at Rio Del Lobo Rock.
That's all Wild Horse Breeds needed to complete the Horseman 10 Challenge in RDR2. For other challenges, check out the Red Dead Redemption 2 Challenges Guide.
All the Horse Breeds in Red Dead Redemption 2 and Where to Find Them
Source: https://www.powerpyx.com/red-dead-redemption-2-wild-horse-breed-locations/