"One Punch Man" recently released a one-shot chapter that explains the Monsters Association. Although it offered some form of explanation on how the monsters are rank, it did not include another level that could pose a threat to humanity.
One Shot Special Chapter
Before the release of chapter 73, "One Punch Man" issued a special manga that could pose a big treat for Saitama and other heroes: Monsters Association. Available in Manga Stream, the chapter offered some basic information on how the monsters are tagged. But since this is Saitama, everything revolves around his goofy character with amusing consequence.
The monsters are classified as the following: Wolf, Tiger, Demon, Dragon. Wolf and Tiger classification refers to the number of persons they could harm - the former is a threat to human life with the latter is a threat to a large number of people. Demon and Dragon classification on the other hand, refers to the cities they could destroy - Demon can destroy an entire city while Dragon monsters could destroy multiple cities in one go.
But the classification, admittedly as its flaw since there are monsters that look weak but actually very powerful. The special issue illustrates the point since the monster featured is basically a human sprout that could freeze anyone at will. But the monster also failed to classify Saitama and he got to his wrong side by freezing him and his baked sweet potato.
Stronger than a Dragon Monster
The special issue has left out one more classification: God. A Reddit post about Monsters Association in "One Punch Man" clarifies the classification of monsters and it doesn't end with Dragon. The God monster's level threat is not just on multiple cities but it's "a threat to destroy all humanity."
If this classification exists, then there could be a monster strong enough to destroy everyone. This monster could arrive soon to challenge Saitama and the Heroes Association.