Mermen, the male counterparts of the mythical female mermaids, are legendary creatures which are human from the waist up and fish-like from the waist down, but may assume normal human shape. Sometimes they are described as hideous and other times as handsome.
Antiquity
Perhaps the first recorded merman was the Assyrian-Babylonian sea-god Ea (called Enki by the Sumerians), linked to the figure known to the Greeks as Oannes. However, while some popular writers have equated Oannes of the Greek period to the god Ea (and to Dagon), Oannes was rather one of the apkallu servants to Ea.
The apkallu have been described as "fish-men" in cuneiform texts, and if Berossus is to be believed, Oannes was indeed a being possessed of a fish head and man's head beneath, and both a fish tail and manlike legs. But Berossus was writing much later during the era of Greek rule, engaging in the "construction" of the past. Thus even though figurines have been unearth to corroborate this fish-man iconography, these can be regarded as representing "human figures clad in fish cloaks", rather than a being with a fish head growing above the human head. And the god Ea is also seen as depicted wearing a fish cloak by modern scholars.
Greco-Roman mythology
Triton of Greek mythology was depicted as a half-man, half-fish merman in ancient Greek art. Triton was the son of the sea-god Poseidon and sea-goddess Amphitrite. Neither Poseidon nor Amphitrite were merfolk, although both were able to live underwater as easily as on land.
Tritons later became generic mermen, so that multiple numbers of them were depicted in art.
Tritons were also associated with using a conch shell in the later Hellenistic period. In the 16th century, Triton was referred to as the "trumpeter of Neptune (Neptuni tubicen)" in Marius Nizolius's Thesaurus (1551), and this phrase has been used in modern commentary. The Elizabethan period poet Edmund Spenser referred to Triton's "trompet" as well.
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.
Medieval period
Marmennill
A merman is called marmennill in Old Norse, attested in the Ladnámabók. An early settler in Iceland () allegedly caught a merman while fishing, and the creature prophesied one thing: the man's son will gain possession of the piece of land where the mare Skálm chooses to "lie down under her load". In a subsequent fishing trip the man was drowned, survived by the boy who stayed behind.
Hafstrambr
The hafstrambr is a merman, described as a counterpart to the hideous mermaid margýgr in the Konungs skuggsjá ("King's mirror", ). He is said to generally match her anthropomorphic appearance on the top half, though his lower half is said to have been never been seen. In actuality, it may have been just a sea-mammal (hooded seal, Cystophora cristata), or the phenomenon of some sea creature appearing magnified in size, caused by mid-range mirage.
Medieval Norsemen may have regarded the hafstrambr as the largest sorts of mermen, which would explain why the word for marmennill ('little mer-man') would be given in the diminutive.
Other commentators treat the hafstrambr merely as an imaginary sea-monster.
Early cartography
A twin-tailed merman is depicted on the Bianco world map (1436). A merman and a mermaid are shown on the Behaim globe ().
Renaissance period
Gesner's sea-satyr
Konrad Gesner in his chapter on Triton in Historia animalium IV (1558) gave the name of "sea-Pan" or "sea-satyr" () to an artist's image he obtained, which he said was that of an "ichthyocentaur" or "sea-devil".
Gesner's sea-devil () has been described by a modern commentator as having "the lower body of a fish and the upper body of a man, the head an horns of a buck-goat or the devil, and the breasts of a woman", and lacks the horse-legs of a typical centaur. Gesner made reference to a passage where Aelian writes of satyrs that inhabit Taprobana's seas, counted among the fishes and cete (, "sea monsters").