You guys be talking about Shibabawa's prophecy and Garou and Murata's version and whatnot, while none of you can see the obvious picture.
Here's my theory. Roast me all you want.
You guys remember those small fries of a monster? Though it says "monster", the kanji itself says "Human paranormality". Anywho, what is one trait that every last of those pricks share? Nearsightedness? Cheap slapstick lines? Unruly appearance and behavior? NO.
The only thing that makes them what the are; they singlemindedly indulge themselves into one thing so much, so dedicated that they become that very thing.
Saitama's first monster, Crabman. Likes crabs so much he becomes one.
The monster who salvages cars and assimilate into them is a car lover.
The chameleon guy loved his chameleon and is always a pervert who wants to have a long tongue.
Not to mention, the Kadokawa guy. He wore his suit, day in day out, that the suit literally grows into him. And that's not all, he even adapts the suit's characterization and achieved pseudo-immortality.
My point is, people are becoming the thing they have spent their lives into. They turn into what they believe as their ideal form. And once they do, they become "monsters". I don't know what makes the difference between regular human and monsters. Maybe their dedication, or it could be their self-suggestion.
Saitama wanted to become a hero who can singlepunchedly kill any kind of monster, and he did. He became the very thing he singlemindedly spent his life into. Is Saitama a human, then? Or does he count as a monster?