A mer-people colony, as seen in Sabrina Down Under.
Mermen and Mermaids (male and female respectively) are beings who appear human from the waist up, but have a large fish tail as their lower body. Mermen/maids are essentially passive creatures. They have the ability to communicate with sea creatures, most notably dolphins.
In "Sabrina Down Under," Sabrina must save a colony of mermen and mermaids from being exposed to the world by Dr. Julian Martin as well as from pollution.
A mermaid is also seen in the season six episode "Thin Ice," in which Sabrina takes on the responsibility of delivering all of Mercury's packages and makes the embarrassing mistake of giving a mermaid a pair of high heeled sandals.
In the season seven episode, "Witch Way Out," Sabrina is imprisoned by a collector of magical beings and finds a mermaid in a cell near her. Sabrina rescues the mermaid and all of the other beings.