ABC's live-action TV series Once Upon a Time is a massive crossover of a show, linking the mundane world we know with the magical worlds of fairytales, literary characters and myths. Since ABC is owned by Disney, quite a few Disney and DIsney-reinterpreted characters show up on the show, although they're all somewhat changed in order to fit into this new universe. The central fairytale of the show's mytharc is Snow White and the Seven Dwarfs, with many others coming in to enrich the canvas including Rumplestilskin, Beauty and the Beast, Little Red Riding Hood, Pinocchio, Peter Pan, Sleeping Beauty, Alice in Wonderland, Cinderella and a whole bunch of others.
The concept of the show is that all these fictional characters are real, and a number of them have been transported from their magical realms into the mundane town of Storybrooke, which exists in our non-magical world. The show mostly follows the adventures of the characters in their present time in Storybrooke, interspersed with flashbacks to their past in the magical lands of their origin.
In fall 2013 the show opened its third season, and mermaids showed up. In particular there's one mermaid whose story intersects with the main characters: Ariel herself, who's played by Joanna García.