Headcanon time! This time I’ll elaborate about my version of mustangs in my MLP AU and Smoky  
 
 
Mustangs originally came about during Grogar’s reign. He needed strong earth ponies for manual labor and no daydreamers- so what did he do? Strip the ability to gain a mark from earth ponies he gathered while simultaneously granting them even more strength than regular earth ponies, of course!  
 
Mustangs are nomadic pony herds, consisting of thoroughbred bloodlines for each herd. They are nomadic because of superstition of Grogar’s return, whom they think wishes to punish them for running from him. There are all sorts of breeds including Mountaineer, Minis (my world’s version of Shetland ponies), Saddle-Arabians, etc; and there are of course more than one herd of each breed usually. Mustangs are usually very closely tied to their herd, protecting each other like one huge family. If a herd member breaks away (which is rare in itself), it’s usually because 1) they get lost or 2) they find a lover outside of the herd within normal pony society.  
 
The mustang ranks goes as follows:  
Herd Leader and Leader’s Broodmare (basically the “royalty” of the herd, consisting of the leader and his mate)  
Leader’s children (speaks for itself, in the wake of something happening to both the leader and his mate passing or going missing, the eldest of their children will take over)  
Scouts (the watch ponies, their job is to keep the herd safe, especially at night)  
Civilian (the regular ponies that don’t fit into any of the above ranks)  
 
 
Smoky is a young mountaineer mustang, the nephew to the current herd leader: Rocky. He’s usually pretty quiet, and quite intimidating just by himself. But, he is also one of the ponies that possesses the rare power known as “The Stare”. Yes, Fluttershy isn’t the only one out there that knows this ability, much like how Pinkie isn’t the only one that has the “Pinkie Sense”. Smoky’s stare is very powerful, so much so that he has to cover his eyes with his hair. This boy is a scout just like his Pa, and he’s a strong fighter- with many a pony in his herd challenging him, where most if not all fail. Going back to herd members breaking away, he is one of the unfortunate souls that ends up torn from his family during a flash flood, and now he has to try to find them again.