According to the IDW comics, Opaline was already alive in G4, probably then she was a young unicorn who had come to Ponyville with a plan in the back of her mind which consisted of harassing Fluttershy and spreading lies to create division and strife among the ponies. Back then, the historical legacy was very heavy on the ponies who still lived in the shadow of a social caste system based on magical ability, despite the fact that this had been demolished many centuries before. In Sunny’s time, there is no more, and she wants to keep it that way in confrontation with Opaline who wants a “perfect Equestria”.
The village that Mistmane came from was populated almost entirely with unicorns, and a G4 pony, Zesty Gourmand from S6 ep. 12 had a straight unicorn horn even though she had an East Asian facial appearance. This shows that Misty may come from the same tribe as Mistmane, Zesty and Opaline. It has never been shown where the village is located, only that it was in a mountainous country with a warmer climate than Ponyville and possibly Maretime Bay in G5, and it was within reach of Rarity.
We know little about the Mysterious South which was visited only once by the Mane 6 in the IDW comics, and it’s possible that there were several trips there during G4 as with the story of Mistmane.
There could be rivers running there to the south and southwest to the savannah lands which could be “the farthest reaches” between the seas in the west and east, and railway tracks there could mean that there were very old communication routes there, especially between South Equestria and the Celestial Sea. The kirins are right there in this strip of land, and when the anugyptian cats had to flee, they came to another continent to the east. It can explain how East Asian culture could spread in the southwestern part of Equestria all the way up to the mountains, and can have its origin in the continent on the other side of the Celestial Sea, Abyssinia is not the only realm there.
But it could be possible that there were very old pony cultures there in the Mysterious South long before Mistmane’s parents moved there, and Opaline could be the result of this. We have nothing about her background, especially how she could become an alicorn if she was a unicorn, and how she could be an evil alicorn without having any signs of corruption on her. All known alicorns have an evil persona as with Luna and Nightmare Moon, Opaline is perhaps the first “normal” alicorn that has no signs of corruption on her, even though she is a villain who has no good plans for the Ponykind. As she is after the pony magic and wants to steal the Unity Crystals and claim her right as an alicorn.
Sunny won’t allow that.