All the stories are from 'Icelandic Legends': Translation of Powell and Magnusson.
THE MERMAN
Long ago a farmer lived at Vogar, who was a mighty fisherman; and of all the farms about, not one was so well situated with regard to the fisheries as his.
One day, according to custom, he had gone out fishing; and having cast down his line from the boat and waited awhile, found it very hard to pull up again, as if there were something very heavy at the end of it. Imagine his astonishment when he found that what he had caught was a great fish, with a man's head and body! When he saw that this creature was alive, he addressed it and said, "Who and whence are you?"
"A merman from the bottom of the sea," was the reply.
The farmer then asked him what he had been doing when the hook caught his flesh.
The other replied, "I was turning the cowl of my mother's chimney-pot, to suit it to the wind. So let me go again, will you?"
"Not for the present," said the fisherman. "You shall serve me awhile first." So without more words he dragged him into the boat and rowed to shore with him.
When they got to the boat-house, the fisherman's dog came to him and greeted him joyfully, barking and fawning on him, and wagging his tail. But his master's temper being none of the best, he struck the poor animal; whereupon the merman laughed for the first time.
Having fastened the boat, he went toward his house, dragging his prize with him over the fields, and stumbling over a hillock which lay in his way, cursed it heartily; whereupon the merman laughed for the second time.