People all over the wiki have theorized about where the entities came from, saying they are the people who died. I am here to extend on that. If you don't know who I am, you can read my profile page.
Essentially, people say that the entities of the backrooms are the people who died here. Other than starvation, dehydration, or sleep deprivation, how could a person die? My theory extends on that thought: The Glitched Goddess made the entities out of her victims.
Here's the evidence: on her page, it says "She created the monsters that lurk these eerie corridors." This likely means she created most of the entities here, but how? Easy answer: reality distortion.
In the same page, it says she can distort reality. So she could easily change the other wanderers' appearance after bringing them back from the dead. They see us as the monsters we see them as. There are a few things this doesn't explain, though, including hybrids.
Hybrids are experiments, either willingly or unwillingly. They become either part entity or full entity. How is this possible? Really, it is possible. She just needs to distort that person enough. For example, Edison drinks smiler water and grows a smiler leg. He is partially distorted, and nothing changes for him. However, Samantha gets turned into a Skin-Stealer, and gets the behavior of one as well. She is fully distorted, and she sees everyone as a skin-stealer. Really, the glitched goddess is a part of the experiments.
What about the other entities, the ones that hate her, or have nothing to do with her? They came from somewhere else, and did not originate in the Backrooms. Some hate her because of what she is doing, while others (strongly in the case of Entity-D) hate her for the power she has, or because she took something from them.
So, that's it. My theory is that the Glitched Goddess made the entities, which are really illusions. Getting rid of the Glitched Goddess somehow may revert these changes she made.