Malcom Douglas --- check this out!

In looking for references to Water Horses, I found a reference to the Shape-shifting Swan maid!

>>>In other cases the old water beings have a more pleasing form, like the syrens and other fairy beings who haunt French rivers, or the mermaids of Irish estuaries. 3 In Celtic France and Britain lake fairies are connected with a water-world like that of Elysium tales, the region of earlier divinities. 4 They unite with mortals, who, as in the Swan-maiden tales, lose their fairy brides through breaking a tabu. In many Welsh tales the bride is obtained by throwing bread and cheese on the waters, when she appears with an old man who has all the strength of youth. He presents his daughter and a number of fairy animals to the mortal. When she disappears into the waters after the breaking of the tabu, the lake is sometimes drained in order to recover her; the father then appears and threatens to submerge the whole district. Father and daughters are earlier lake divinities, and in the bread and cheese we may see a relic of the offerings to these. <<


The full Article can be found HERE


So maybe my friend's informant wasn't mixing Universal Horor film mythology with an Irish song. Maybe she was relating a story which sprung from an older tradition. Pretty neat, eh? I went looking for a horse and found a swan.