Mermaids

Mermaids are a humanoid species featured on ABC's Once Upon a Time. They are native to Neverland, and first appear in the first episode of the third season.

History
The Queen of the Enchanted Forest, Regina, goes to Rumplestiltskin's castle to discuss with him a deal with a particular mermaid. The conversation is cut off by Rumplestiltskin asking her to leave, to which she follows through with.
 * -|Before the Curse=

As party members on the Jolly Roger, David, Emma, Hook, Mary Margaret and Mr. Gold use a magic bean to enter the world of Neverland and rescue a kidnapped Henry. After Mr. Gold parts to Neverland's island to continue the search on his own, the group run into trouble with a swarm of mermaids, who are attacking the ship profusely from all sides. Only Hook is unsurprised to see what they are facing, and as he tries to steer to outrun the overwhelming horde, David fires ammunition from a cannon into the water repeatedly to force some of the creatures to fall back. On the other hand, Emma and Mary Margaret use a fishing line to ensnare and reel in a mermaid. Regina, seeing that both efforts are not producing the necessary result, takes matters into her own hands and conjures fireballs; throwing them into the sea, which scares off the remaining school of fish into dispersing. Noticing the weight of the mermaid on the fishing line is struggle to pull up for Emma and Mary Margaret, Regina hauls and teleports it onto the deck.
 * -|After the Curse=

Disgusted, Hook calls for the mermaid to be thrown off his ship, but Regina disagrees as it might be helpful to have a hostage. David sides with Hook and wants the creature away from them. Regina proposes they should find out why the mermaids were attacking while Mary Margaret wonders if she means to torture it, to which the latter is open to. Sneakily, the mermaid grabs a conch shell that was also tangled in the fishing line and blows into it to signal a warning; eliciting the sound of thunder from the sky as a response. She threatens them all with death if they don't free her. David attempts to interrogate the mermaid about what exactly she did by sounding the shell, while the half-fish half-human being seductively pleads to be let go. Regina refuses, and warns she will use force if no answers are given. Frustrated, Mary Margaret states threatening someone isn't a way to get a response, though Hook clues everyone in that a mermaid's word absolutely cannot be trusted as they are all liars. Mary Margaret suggests perhaps the mermaid is afraid of Pan and letting her go would be best, but Regina hints if that happens, it's just as likely the whole horde of them will be swimming right back to kill everyone. With an air of confidence, the mermaid sneers she doesn't need her friends to slaughter them, and they will do it themselves.

Unexpected loud thunder claps brewing from the sky startle everyone; making Hook realize the mermaid previously called the storm to arms. Using authority with a sword, David presses the creature to stop the storm, and then they will let her go. She refuses, and an impatient David moves to prepare slashing open her neck as Regina incites him to go through with it. However, a shocked look from Mary Margaret deters him away from doing so. During Hook's struggle to motion the ship into speeding away from the storm, more bickering follows Mary Margaret and Regina's disagreements over what to do with the mermaid. Eventually, Regina decides for the group to freeze the water nymph into a wooden statue so the storm disturbance will cease, but instead, speeds it up into a downpour of rain and gigantic sea waves. Much later, Emma discovers the worsening of the storm is actually the result of everyone's animosity towards each other, and risks her life to bring the group into working together; thus helping to calm the sea as well.