Normal mermaids also appear in "The Great Bird Man." In this appearance, several of them are depressed and want to beach themselves. Princess Bubblegum discovers they are just lonely, and Finn promises to hang out with them after his mission. Xergiok later gets into a relationship with one, improving both of their moods.
Mermaids are female humanoids with the tails of sea creatures and other aquatic appendages on their humanoid half. The skin, hair, and tails of a mermaid vary between them.