How did they make the merman who becomes a mermaid when he changes?

I’ve only encountered one mermaid thus far but I haven’t really explored Sulani yet. Nalani who I think comes with the pack. I moved her into a beach house. She has the tail in the water or bath but legs on land!

I’ll keep a look out for the merman and if I can, move him in and look at his info. Perhaps there’s a way to assign mermaid abilities to male Sims?