№7345[Reply]
Your personal MLP headcanons? I'll start:
Equestria is a very big nation that was for most of its history severely divided and culturally incompatible with each other (Much like Italy before Giuseppe Garibaldi and Germany before Otto von Bismarck).
Likewise, a diverse array of dialects and accents flourished as these kingdoms, princedoms ducats, and even short-lived Republics struggled for power.
One of these accents, one that sounded alike Southern English, was developed around the prestigious Queendom of Canterlot, which is historically the most politically influential and powerful nation among the surrounding countries that were in all but name vassals of Canterlot, though the mountains and Canterlot itself were mostly inhabited by Unicorns, beyond its barbicans there were vast portions of plain, arable terrain all around, where Earth Pony farmers worked in a Free-Market based economy, competing with other farmer families in order to win over wealthy Unicorn clients (That would explain why Filthy Rich, an earth pony, has such a strong kinship to trading and business, and why the Apple Family had that an inflamed rivalry with the Pear Family, it was basically tradition to compete among Earth Pony Farmers).
Since Equestrian society was based on Free-Market competitions since the very start as opposed to the medieval feudalism, Earth Ponies often traveled to Canterlot both for business and for fun, but those were the rule. The exception was the migrants, some earth ponies farmers simply desired something more than farming and being the benefactors of Unicorns, eventually, these Earth Ponies were integrated into Canterlotian society, which lead to these Earth folk to speak the british-like accent the unicorns spoke in Canterlot.
And that's why Octavia has a british accent!