Louis' father, or the King of their small, costal country, demands a pet mermaid. A beautiful one, one that would add color to the large, decorative pool they have in the middle of the palace. The only problem is that the mermaid doesn't want to be there. No, he wants to be free, with all of the other merfolk. But the prince, the prince does everything he can to make the poor creature smile. And maybe the creature makes him smile as well.
Or, a larry au where Harry is a mermaid, Louis is a prince, and shit happens.