Sunset Shimmer thought she was free and clear for the next 30 moons. She'd timed her trip through the portal so she'd transit at night, and it turned out (or so she thought) to work perfectly! Nobody saw her come through, which meant she was able to start learning how to pretend to be one of these creatures called 'humans.' She learned she resembled an adult woman, though perhaps on the young side (people were surprised when she told them she'd graduated university already, which told her even in this new world she was already something of a savant), found an abandoned motel in good condition that she'd social engineered her way into squatting, got a job at a coffee shop to pay for her lifestyle, and settled in to wait.
She was pleasantly surprised to discover that humans weren't, after all, the only intelligent creatures on this side of the portal, there were a whole catalog of peoples known colloquially as 'monsters' that had just announced their presence and had started the Interspecies Exchange Act, administered by the Interspecies Cultural Exchange agency. While interesting, she didn't think anything of it...
...until an agent knocks on her door to introduce Sunset's new roommate, a lamia named Rarity.