The Sea-Maiden is a Scottish fairy tale collected by John Francis Campbell in Popular Tales of the West Highlands, listing his informant as John Mackenzie, fisherman, near Inverary. Joseph Jacobs included it in Celtic Fairy Tales.
A mermaid offered a fisherman much fish in return for his son. He said he had none. In Campbells' version, she offered him grains: three for his wife, three for a mare, three for a dog, three to plant in the yard; then there would be three sons, three foals, three puppies, and three trees, and she should have one son when he was three. In Jacobs's version, she merely said he would have a son, and when the boy was twenty, she would take him.
In Campbell's version, the mermaid let him put her off until the boy was twenty.
In both, the father grew troubled. The son (or oldest son) wormed the problem out of him, and told him to get him a good sword. He set out on horseback, with a dog, and came to where a dog, a falcon, and an otter quarreled over a sheep carcass. He split it up for them if they came with him and aided him.
He took service with a king, as a cowherd, and his pay was according to the milk. Nearby, the grass was poor, and so were the milk and his wages, but he found a green valley. When he pastured the cows there, a giant challenged him for grazing in his valley. He killed the giant. Taking none of its treasure, he took back the cows, which gave good milk. The next day, he took the cows further and had to fight another giant, with help from the dog. The third day after that, he took them still further and met a hag who tried to trick him, but he killed her with the help of the dog.
When he went back, everyone was lamenting. A monster with three heads lived in the loch, and got someone every year; this year the lot had fallen to the king's daughter. The general said he would rescue her, and the king had promised to marry him to the daughter if he did. The son went to see. When the monster appeared, the general ran off. The princess saw a doughty man on a black horse, with a black dog, appear. He fought the creature and had off one head, drawing a withy through it. He gave it the princess, who gave him a ring. He went back to his cows, and the general threatened to kill her if she did not say that he did it. The next day, the king's daughter had to go back, because there were two heads left. The son came again and slept, telling her to rouse him when the creature came; she did, putting an earring of hers on his ear as he said, and they fought, and he cut off the second head. The same thing happened the third time, and the creature died.