I assume that the world in which My Little Pony takes place is NOT called Equestria. Instead, I assume the MLP world is called Ponyworld, with the kingdom of Equestria constituting only part of Ponyworld. I also assume that the world of Equestria Girls, which is dominated by humans (sort of), is NOT called Earth, but rather named Humanworld.
Ponyworld, I assume, is flat like Discworld from Terry Pratchett's novels or the flat Earth as envisioned by the Flat Earth Society. Ponyworld's sun, which is raised and lowered by Princess Celestia, is not a really star made of hot gases like Earth's Sun, but made of magical flame. Ponyworld's moon, as raised and lowered by Princess Luna, is actually made of rock, possibly some type of moon stone.
Humanworld is a spherical planet like our Earth that orbits a real star like Earth's Sun, and likely has a natural satellite called a moon that actually orbits Humanworld. Humanworld is a different planet from our Earth, being in different universes. Moreover, Humanworld humans and Earth humans are different intelligent species.
The larger universe that contains Equestria and Ponyworld likely has many other flat worlds like Ponyworld, with their own magic, races, suns, and moons. The so-called stars seen in the Equestrian night sky are actually the suns close to worlds other than Ponyworld.
Humanworld likely occupies a solar system with other planets with their own moons, and this planetary system occupies a galaxy with other stars and solar systems, and the universe containing Humanworld likely has other galaxies.
Of course, the Crystal Mirror, is one way to travel between Ponyworld and Humanworld. But I speculate that in relatively rare cases that natural phenomena like hurricanes, tornadoes, waterfalls, and whirlpools might actually see inhabitants accidentally (or maybe deliberately?) travel between Ponyworld and Humanworld.
Star Swirl the Bearded had his own magical mirror permitting travel between Ponyworld and other worlds, and he probably used his mirror to banish The Dazzlings to Humanworld. Of course, his mirror was likely lost after star Swirl died.
Of course, the experiences of Twilight Sparkle and Sunset Shimmer suggest that any pony leaving Ponyworld for Humanworld automatically becomes human. And any Humanworld human that ends on Ponyworld becomes a pony.
Reminding myself of a well-known TV series by Sid and Marty Krofft, I envision a Humanworld family on summer vacation in a rubber raft ending up falling over a waterfall or into a whirlpool, ending up in Ponyworld and Equestria, transformed into ponies. How about "Land of the Ponies?"