Humans in mosters guise
There is a very common trope that monsters are humans in disguise.
It is true.
Most of the time this monsters just wander around aimlessly watching things happen, develop and learning. They will try to adapt and mimic human behaviour until they feel comfortable enough, some of them even admire us, love us. Others just blatantly hate us, while others just want to have fun, you know, like the pop song, "monsters just wanna have fun".
We can not see them, or at least can no longer see them, many factors come into play, pollution, lighting, humidity, but mostly, people forget them and don't care to see them.
This changelings, wander around, they like what we have done with the planet, they like shinny things, travel by poisonous gasses and love to play with fire. Once a changeling said, hey, I like humans, I think I will become one of them, and he did, he followed around a fat human, he knew fat humans were the most comfortable ones and guessed that their skin would have enough room to breath.
He waited until he was alone, it did not take much time, for he was a rich lonely man. He struck and ate him, the man felt the claws, the fangs, he screamed, he begged, he never knew what attacked him. The monster took his place, he thought he knew what to do with that new human body, his family, the children that were away but loved him found a different man, they rejected his monstrous ways and went away again to never see him again. The monster on the other hand clamoured to his invisible friends, people around him thought him eccentric, that is the fancy word for crazy, but no one did nothing, one by one, other monsters followed his lead, they found a rich man or woman's body and took their place.
One thing they did not account for humans, is that humans have mortal bodies, and so, by clinging to their meat, bones and skin, they lost their ethereality and immortality.
The human flesh withered and rotted, eventually the man died. Again. The monster soul forever trapped in a decomposing body. The other monsters did not realise at first, the jail in which their friend was trapped, but as the flesh died, the monsters realised their mistake, flesh was a prison for them, afraid of losing themselves, trapped, and latter buried or burned they tried to find ways to "fix" mortality. Cryogenics, rejuvenating treatments, pills, drugs, wasting resources in every way possible and not caring about anything but themselves to keep on living. Futile.
Monsters will wither away. Among and in the human bodies they grew to hate. Some day, new generations will fix their undoings, until then, we just have to fight them and make their lives as shitty as possible, with the love and wittines this monsters don't understand.