Jamieson's Northern Antiquities gives the story of the most famous of the Crodh Mara, the cow bred by the visit of a water-bull and of the farmer too mean for gratitude.
The elf-bull is small, compared with earthly bulls, of a mousecolour; Mosted [crop-eared], with short corky horns; short in the legs; long, round, and slamp [supple] in the body, like a wild animal; with short, sleek, and glittering hair, like an otter; and supernaturally active and strong. They most frequently appear near the banks of rivers; eat much green corn in the night-time; and are only to be got rid of by certain spells .
A certain farmer who lived by the banks of a river, had a cow that was never known to admit an earthly bull; but every year, in a certain day in the month of May, she regularly quitted her pasture, walked slowly along the banks of the river, till she came opposite to a small holm covered with bushes; then entered the river, and waded or swam to the holm, where she continued for a certain time, after which she again returned to her pasture. This went on for several years, and every year, after the usual time of gestation, she had a calf. They were all alike, mouse-coloured, mosted, with corky horns, round and long-bodied, grew to a good rise, and were remarkably docile, strong, and useful, and all ridgels. At last, one forenoon, about Martinmass, when the corn was all 'under thack and raip', as the farmer sat with his family by the ingleside, they began to talk about killing their Yule-Mart. 'Hawkie,' said the gudeman, 'is fat and sleek; she has had an easy life, and a good goe of it all her days, and has been a good cow to us; for she has filled the plough and all the stalls in the byre with the finest steers in this country side; and now I think we may afford to pick her old bones, and so she shall be the Mart.'
The words were scarcely uttered, when Hawkie, who was in the byre beyond the hallan, with her whole bairn-time, tyed by their thrammels to their stalls, walked out through the side of the byre with as much ease as if it had been made of brown paper; turned round on the midding-head; lowed once upon each of her calves; then set out, they following her in order, each according to his age, along the banks of the river; entered it; reached the holm; disappeared among the bushes; and neither she nor they were ever after seen or heard of. The farmer and his sons, who had with wonder and terror viewed this phenomenon from a distance, returned with heavy hearts to their house, and had little thought of Marts or merriments for that year.