Merpires: they have the hydration need and have a tail in their dark form's swimsuit section. They have he mood songs but can't control weather. They can do vampire things like drink blood, are weak to the sun, and have a smaller perk tree. But they don't inherit weaknesses.
Vampilien: Vampire mermaids with an alien form. The disguise has access to dark mode vampire resources. They get all alien powers, are not harmed by sunlight while disguised, and get a few vampire abilities related to controlling other sims. No perk tree, but they can still drink blood and turn other sims into vampires. Gain positive moodlets at night.
Mermaliens: All the powers and abilities of mermaids and aliens. Their true form is an alien with a mermaid tail. Extra flammable because why not. Get sick from eating fish, as their species can't handle them