Yeah, basically take a monster from a legend/mythology, either well known or not and attribute why they’re in the human world, either why they got banished, exiled or perhaps they were simply a monster who couldn’t travel to the monster world or heck maybe they’re a hybrid of some kind!
Here’s an American one:
Emogoalekc.
(The myth):
Rumoured to have once been human. He fell in love with a slave girl as the son of a chief but was forbidden to marry her by his father. Upset, the boy then jumped into a lake and was transformed into a monster instead of dying.
A close friend came to him and recognised him despite his different appearance. He was sworn to secrecy.
However the truth came out and fearful the tribe decided he might take in his head to attack him, even though he had no interest in doing so. At first he thought his friend had betrayed him but then realised someone else had figured it out and he hadn’t. His friend helped him to escape the villagers and also he gave his still-human friend a promise that if anyone could catch sight of him they would become a chieftan themselves.
So how could you make this fit with M.I. if you wanted to? A part of me thinks the boy was always a monster but had morphing capabilities he found difficult to control- perhaps being a hybrid of some form. He perhaps was descended in some form to the monsters who never left the human world. In away he knew he couldn’t be chief and thus set up a way a chief could be chosen later. (Maybe his particular human friend or their descendants.)
Feel free to tell me if this idea is stupid or something, I’m just interested in the things people might come up with in terms of old myths and stories about monsters.