(Edit Later) (Beta this please)
Mythical Beasts are different from hybrids, as they are born into a normal animal family as an animal. They won't stop growing, and end up being extremely large in animal form. When they peak adulthood, they have an alternative human form, no one knows why but it's assumed that it's because humans are at the top of the food chain.
Mythical Beasts were worth a lot when hybrid trafficking was around, as animal-form skins could be sold for millions of won and they could sell them as pets.