When the universe was only just coming into existence, everything still had a somewhat “glitchy” structure. The main part, where most beings live, managed to form properly. However, a small fragment seemed to fall out of it, separating itself and becoming something like a world of its own.
Considering that this universe does not function entirely normally, malfunctions occurred quite often. For example, objects, creatures, entire pieces of land from the main world, and even certain chemical reactions could end up in this pocket. Most of the time, everything simply fell into the abyss, but eventually something resembling stable ground managed to form there, and everything else began to land on it.
In this way, little by little, an environment started to emerge. It is worth noting that things from all across the universe were discarded there, so the presence of something not native to Earth is nothing unusual.
Why the name “pocket”? Because I associate it with a place where things fall in and disappear, just like into the pocket of a pair of jeans. I have no idea why you would need this information.