Well, as you may know, as children, we imagine scary creatures that hide under our beds or in our closets. Well, these monsters are real. The "monsters" were here before you or whoever lived there before you.
This piece of land is their territory and this house is on their land. So, they make themselves comfy. They hide under beds and in closets to simulate the caves they used to live in. Some of these creatures have even become famous through various creepypastas. Some of these include B.O.B., the Rake, the Seedeater, Mr. Widemouth, and even Slenderman.
In reality, they are not frightening or even inherently aggressive, they just protect their homes from intruders. They're actually more scared of us than we are of them. Some humans make friends with these creatures. Some are even sent here as young babies disguised as humans. How do I know this? Because I know some people who are.
So, next time you check under your bed or in your closet, remember, it's their home. Don't disrespect them. Don't antagonize them. Be respectful of their property. If we're scared of them or think of them as "monsters," remember, to them, we are the real monsters.