In my eyes, it is possible for it to exist in a different universe. I have 2 reasons for this, both based on the idea of infinite universes (multiverse)
1. Big Bang: This would work if the universe split to allow every possibility to happen. Say that Earth never happened. There is a universe for that. Not a fan of Mars? There is a universe for that to. With this idea, a universe could exist, almost identical to our own but with Equestria existing on Earth.
2. Creating Deity: Another way to look at it could be if the universes were created by a living organism. This theory is that every time a living thing must make a decision,the universe splits to create a universe in which every possibility is catered for. Thus the "Creating Deity" would have had the choice to create an endless amount of universes containing Equestria.