Mermaid

Mermaids are a minor recurring theme in The Legend of Zelda series. While mermaids themselves have, to date, only served minor roles in the series, the idea of mermaids has somehow served a larger role (Mermaid's Cave, Mermaid Suit etc). It is noteworthy that no real mermaids have ever appeared; the only one actually met was part of the Wind Fish's dream.

Link's Awakening
Mermaids first appeared in. During his journey around Koholint Island, Link encountered Martha, who had recently lost her necklace to the sea. By an act of chance, Link had found her necklace, and so gave it back to her. In return, Martha allowed Link to take a scale from her tail. Although not necessary to the story, helping Martha is undeniably useful in solving the puzzle of the Wind Fish's Egg.

Oracle of Ages
Although no mermaids appeared in, there were references to them. These were all connected to the sixth dungeon: Mermaid's Cave. To access the full dungeon, while traversing time, Link needed two keys, the Old Mermaid Key and the Mermaid Key. The treasure he would find in this dungeon would also turn out to be the Mermaid Suit.

When Link arrives at Zora Village, a Zora acts completely unsurprised to see a mermaid, as Link appears at the time. Although it is not explicitly stated, this may suggest that mermaids are common around Labrynna.

Phantom Hourglass
In another minor role, mermaids are referenced in Phantom Hourglass in a sidequest. However, this mermaid is a fake, a girl named Joanne who simply loves to dress up like one; she ends up fooling a man who says that he would love to see one.