Once Upon A Time, there were only three mermaids left of their kind. They were living happily until these horrible creatures.
There were only a few monsters so it wasn't really a threat, but they produced monsters quickly.
Humans started seeing monsters at their local beach. These monsters are too powerful for just three mermaids.
There were monsters everywhere. I mean everywhere. They just kept spawning.
A fairy godmother summoned and granted these three lovely mermaid a wish. She couldn't get rid of all the monsters, but they the fairy god mother could turn them into humans. They could go far far away and never have to be bothered by monsters again.
They accepted the wish and lived happily ever after as humans.