>>2899530>>2899630The only conceivable way they could remain secret from humanity from the dawn of mankind until 2009 AD is with some sort of magical world-separating akin to pocket-dimensions.
The Monster Musume universe is already overtly confirmed to contain outright supernatural elements and entities, so this is not only possible but probable.
Mythic species retreated into supernatural segregation from the physical Earth and mankind, emerging only to secretly trade for needed resources (including human men).
These boundaries were eventually discovered and overcome by human research, likely by a governmental power.
Then for an unknown amount of time, governments kept mythical entities being real as a secret from the majority of human society, before making a public revelation in 2009 and beginning integration.
The use of supernatural portals is also ALREADY how it works in Okayado's 12 Beast, so it's completely conceivable and even likely that Monster Musume works in the same way.
