In Greek mythology, mermen were often illustrated to have green seaweed-like hair, a beard, and a trident. In Irish mythology, mermen are described as extremely ugly creatures with pointed green teeth, pig-like eyes, green hair, and a red nose. In Finnish mythology, a merman (vetehinen) is often portrayed as a magical, powerful, handsome, bearded man with the tail of a fish. He can cure illnesses, lift curses and brew potions, but he can also cause unintended harm by becoming too curious about human life. The boto of the Amazon River regions is described according to local lore as taking the form of a human or merman, also known as encantado ("enchanted one" in Portuguese) and with the habit of seducing human women and impregnating them. Chinese mermen were believed to only surface during storms or, in some cases, were believed to have the ability to cause storms.
The actions and behavior of mermen can vary wildly depending on the source and time period of the stories. They have been said to sink ships by summoning great storms, but also said to be wise teachers, according to earlier mythology. A merman, like a mermaid, attracts humans with singing and tones.
- The most well-known merman was probably Triton, son of Poseidon and Amphitrite. Although Amphitrite gave birth to a merman, neither Poseidon nor Amphitrite were merfolk, although both were able to live under water as easily as on land. Triton was also known as the Trumpeter of the Sea for his usage of a conch shell.
- Other noteworthy mermen were the Babylonian Oannes and Ea, and the Sumerian Enki.
- Another notable merman from Greek mythology was Glaucus. He was born a human and lived his early life as a fisherman. One day, while fishing, he saw that the fish he caught would jump from the grass and into the sea. He ate some of the grass, believing it to have magical properties, and felt an overwhelming desire to be in the sea. He jumped in the ocean and refused to go back on land. The sea gods nearby heard his prayers and transformed him into a sea god. Ovid describes the transformation of Glaucus in the Metamorphoses, describing him as a blue-green man with a fishy member where his legs had been.
- Norse mythology, in particular Icelandic folklore, has mermen known as Marbendlar.
- In Dogon mythology (not to be confused with the semitic fish god Dagon), ancestral spirits called Nommo had humanoid upper torsos, legs and feet, and a fish-like lower torso and tail.
- The Russian medieval epic Sadko contains a Sea Tsar who is a merman.
- In Indian mythology, Merman is the first incarnation of Lord Vishnu, who has lower part of the body like a fish, and upper part like that of a man. He has four arms; with two hands he holds a conch shell and wheel, while the other two are holding a Lotus or a Mace.
In some modern works and films however, mermen are portrayed as handsome, strong and brave. They are almost always portrayed wearing no clothing, no matter what the temperature of the water is, with the exception of armour or cloaks in some settings.
- King Triton is a prominent character in Disney's The Little Mermaid franchise.
- The Thirteenth Year (1999) is about a teenage boy turning into a merman.
- In the DC Comics mythology, mermen are a common fixture of the Aquaman mythos, often showing a parochialistic rivalry with humanoid water-breathers. Their origins are described in the Atlantis Chronicles, the metafictional collection of atlantean lore. When Orin the First, ancestor of Aquaman and first king of Poseidons (an underwater city born after the submerging of Atlantis) perfected a serum to turn human beings into water-breathers (the current humanoid water-breathers, ancestors of Aquaman himself and his people), his sorcerous brother Shalako spitefully tampered with the serum, using dark magic to turn his fellow citizens of Tritons into mermen and mermaids. One peculiar merman, the villainous Kordax the Accursed, Shalako's nephew, exhibits a strength exceeding his brethren and the ability to telepatically commune with water life, somehow passing both gifts to Aquaman, making him a superhero and an outcast at the same time.
- Erotica author Megan Hussey has written several romantic e-books pertaining to the merman legend, including "A Good Catch" and "Prince of the Seas," released through Silk's Vault publishing, as well as "Azure Masquerade" and "Passion Storm" through Midnight Showcase (in print, audio and e-book versions).
- The Creature from the Black Lagoon could be seen as a modern adaptation of the merman myth.
- One of the main villains in the He-Man and the Masters of the Universe animated series was named Mer-Man; he was the lord of aquatic beings of similar appearance.
- In an episode of Family Guy, after falling overboard from a cruise ship, Lois Griffin is saved from drowning by a merman who, in reverse of tradition, has the upper body of a fish and lower body of a man. The merman, believing all women fantasize about making love with mermen, is incensed when Lois rebuffs his advances, despite his use of logic in stating that the traditional merman would be unable to have sex. The merman attempts to force himself upon Lois before she pushes him down and leaves him flapping helplessly in the sand.
- In Kamen Rider Kiva, Bassha is a Merman who takes a form of a young boy, and provides one of Kiva's weapons. He can spit deadly water bubbles.
- Matthew Arnold's poem "The Forsaken Merman" is based on the story of a merman marrying a human.
- Jimi Hendrix sings about himself escaping from a war torn world with his love Catherina and becoming a merman in the sea in his song "1983... (A Merman I Should Turn to Be)", from the psychedelic album Electric Ladyland by The Jimi Hendrix Experience.
- In season 3 of Eastbound & Down, Kenny Powers pitches for the Myrtle Beach Mermen and the baseball jersey depicts a cartooned mythical creature
- In The Cabin in the Woods, a merman is a possible "cellar option" being consistently bet on by Hadley, and is featured attacking Hadley during the final scene.