The so-called Joyful Cave is exactly what it sounds like.
It's a very unhealthy place.
It's unhealthy because the authentic Joyful Cave is filled with female merfolk.
Naturally, the East Sea has seas, and there are merfolk in the seas.
Regarding the classification of merfolk, there has always been internal disagreement within the Beast Taming Sect.
If you call them part of the Demon Race, they aren't smart enough and are too bloodthirsty.
If you call them Demon Beasts, they can transform right after birth.
If you call them Spirit Beasts, they can't be properly domesticated.
Later on, the Beast Taming Sect abandoned classification, and merfolk became a unique species on their own, categorized under Sea Race.
After all, there are a lot of unknown things in the sea, and these disorganized, bizarre creatures are all collectively referred to as Sea Race.
