You are the First mermaid in a world of monsters, and magic is unknown. You are truly one of a kind.

This world does not know monsters; it does not know magic, nor does it know any oddities. You are the first how you came to be, and everything else is entirely up to you, but now here you stand and outlier in a world of the ordinary: the First mermaid. I don't believe it would be all that interesting; we're only humans, so there are humans, elves, dwarves, goblins, and orcs.

(Feel free to recommend and suggest other creatures and entities that you would like to be the first of.)