There's a couple things I'm making some logical assumptions from. In the first two episodes, you meet Princess Celestia and her little sister Princess Luna. Now, Luna got jealous, and was imprisoned in/on the Moon for a thousand years. She has these bat-winged pegasi for her guards. The logic I am using, is that if she was imprisoned there, then it must be habitable. If it is habitable, then it would (using ponyverse biological/astrophysics) logically have its own pony population. Most likely, there would be Moon Ponies (analog to earth ponies), Moon Unicorns (analog to unicorns in Equestria) and Night Pegasi (analog to day-pegasi). Now, for obvious reasons, the moon ponies and the moon unicorns aren't going to be flying down to Equestria. But the Night Pegasi would be able to, just most of them don't *want* to go exploring in such dazzlingly brightly lit places. But a curious Night Pegasus could possibly go exploring Day Pony lands to learn about their Equestrian kin, perhaps distantly related (millennia ago, all ponies were Equestrian, and some migrated to the moon at one point) and they diverged, like the Zebra ponies are from other lands, but they're still ponies.
Now, since Night-Pegasi can fly, they could easily enough travel back and forth, but the bright light is far more than they're used to, so they generally don't unless their princess, Princess Luna, asks them to. It's not that they can not, it's that they do not wish to because it is so bright. So now that Luna is welcome in Equestria again, perhaps a few, very bold, very adventurous Night Pegasi might come visiting. Not as guards, but as students and explorers who want to learn about day ponies, and let day ponies learn about them. Even in the canon cartoon, day-ponies of all types get scared by ponies who are different, so it is possible that this could be done. But this thread is all for speculation and discussion too.