From what I've read and remember in-game, there's nothing to really point to the regional origins of the various stable owners. They're just Hylians with a name and similar appearances, but that's it.
theyre probably like from the local area....
I just started a new BotW game so I can visit the stables and check the dialogue of each stable employee. Here are my findings:
Blynne says: "I grew up taming horses 'round these parts. I'm confident there's not a rider in Hyrule better'n me." He is located in the Highland Stable, near Fural plain, a place filled with horses.
In the same stable,
Perosa says that nomadic tribes used to gather there until raiders took over the plains. Considering the highland stable is very close to the Horse God's fountain, it is possible that these nomadic tribes originated from Faron, especifically the Fural plain. It would explain their close connection with Malanya.
Rensa states "It might be hard to imagine now, but this place was nearly destroyed about 100 years ago. However, thanks to the blessings of the Dueling Peaks, everything is full of life again. Even the wild horses are back. There's a lot of energy in this place!" Nearby is Blatchery Plain. Perhaps these nomadic tribes roamed the bigger plains of Hyrule and periodically gathered in the Fural plain to be near Malanya? perhaps an annual festivity of sorts or to trade and catch up.
Sidenotes:
The two stables located in the Fural and Blatchery plains are better stocked and have fences
Several employees of the stable don't seem to be part of the nomadic people that own the stables, one in particular,
Trott, speaks of wanting a stable of his own, but his employers are overworking him.
Edit: Finished all the stables. None of the employees say anything remarkable about the stables, except for Aya, from East Akkala Stable, who states that her life became dull after marrying into the family.
Conclusion
While its not explicitly stated from what region they are from, the game suggests that these nomadic peoples lived mainly in the plains of Hyrule and periodically gathered at the Fural plain, close to the Horse God.
