Pretty much you'll find them being vendors and they aren't all nice either. One vendor, I think it was during the one where you go to shop and swap collectibles, (can't remember the name, fog brain this am), was mean and several other "negative" traits. I almost always get the reward that let's you see what traits are upon meeting Sims so I spotted her right away AND it told me she was a mermaid. Very, very useful reward, especially when spouse hunting and for only 500 points so worth it early on. The second vendor I approached also was a mermaid and I got her to move in by doing several cloud watching interactions till the "Ask to move in" option showed up. She is more difficult to control though I was able use the mermaid song and mermaid kiss with her.