Some theorists hold that the monsters in dungeons are all adventurers who went to sleep and, when they woke up, found themselves transformed.
This is said to explain why more powerful monsters tend to live on the deeper dungeon levels, when one would expect them to seek the sunlight. Greater heroes, the theory says, will tend to explore deeper, and become more dangerous monsters.
(Author's Note: I have misplaced my notes, and cannot recall where I saw the initial research that inspired this post. Can anyone help?)