The general characteristics of a mermaid are clear and well defined. They date from times of great antiquity and have been retained unaltered almost to the present day. According to this set of beliefs, the mermaids are like beautiful maidens from the waist upwards, but they have the tail of a fish. They carry a comb and a mirror and are often seen combing their long and beautiful hair and singing with irresistible sweetness on some rock beside the sea. They allure men to their death and their appearance is ominous of storms and disasters. According to this set of beliefs, mermaids are not only ominous of misfortunes but actually provoke them, and are avid for human lives, either drowning men or devouring them. In some of the early Celtic descriptions they are monstrous in size, like that recorded with some detail in The Annals of the Four Masters. She was 160 feet in length, her hair eighteen feet (comparatively short), her fingers were seven feet in length, and so was her nose. These exact measurements were possible because she was cast up by the sea. This was said to have happened in about a.d. 887.

Some sea monsters, such as nuckelavee, were allergic to fresh water, but mermaids penetrated up running streams and were to be found in fresh-water lakes. They were still called mermaids. A typical example of a ravening mermaid is to be found in the story of 'The Laird of Lorntie' told by Robert Chambers in Popular Rhymes of Scotland: