What if?
Rules:
1. The monsters will not look like a human or even similar to a human so no vampires, orcs, elf's, or mermaids. (However robotic humans like Ultraman or Evangelions are exceptions)
2. No God like monsters so monsters that can destroy an entire universe, planet, moon, or continent are not allowed. The monster has to be at the very least nuclear, city, or large island level.
3. Physics would be ignored because lets be honest most non human monsters like dragons or kaiju would die from their own weight or to be big to barley fly or even move.
4. Everyone would keep their intelligence and speak their native language despite being completely different from humans.
5. Everyone would into transform to a monster based on their personally and or desires; for example, a person who likes eat and gets angry really easy would transform into a Deviljho or any other monster that fit this description, while a person who very possessive and protective over people or property would transform into a Diablos or any other monster that fit this description. Another example is someone really likes Pokémon will become one in this scenario.
6. Everyone will be able to transform back into human and monster but it will take some time to figure out how to.
7. These people would be able to pass down this strange transformation trait to their children.