The Xana are water spirits found in Asturias mythology, and they're pretty much a blending of every generic water spirit you've ever heard of and good old fashioned fairy motifs.
She's always female and lives in freshwater areas. She's got the siren voice going on, as well as the Disney's Peter Pan, 'sitting by the shore and brushing long elegant flowing tresses' mermaid thing. Unlike most water spirits, Xanas don't seem all that interested in dragging innocent people down to a watery grave. They're a rather benevolent spirit, all things considered, and will sometimes give lost travelers directions and safe water to drink. The only thing they really have going against them is the whole stealing human children thing.
Oh yeah, they do that. That's where the traditional fairy motif kicks in: Xanas are unable to produce milk for their own babies, so they switch them out with human babies, leaving the unsuspecting human mother to raise the Xanin(os) for a few months. After that, the kid is fully grown.
The one way to tell for sure your kid is a Xanin (other than consulting a pediatrician about why Junior is already able to walk and talk at three weeks old) is to do something to shock it into admitting it's a Xanin. (The usual method done in many changeling stories is to, say, maybe drink wine from a boot or cook soup in an eggshell. Then they'll usually say something along the lines of, 'I was born in INSERT NAME OF RIVER/INSERT RIDICULOUS NUMBER OF YEARS AGO and never have I seen such a thing as that!")
Oddly enough, there seem to be some discrepancies among the Xana themselves. The ones who are good and wholesome look like pretty young women, where as the child stealing ones are usually smaller, uglier, and more prone to other malicious assholery. Those ones get into your house by turning themselves small and going through the keyhole, or turn into rats and come in through the floor. Because as we all know, goodness is always pretty and badness is always ugly. As to what happens to the human kids. . .
Hey! Did I mention the Xanas sometimes have treasure? And they can shapeshift? And in some stories you have to help them with whatever ailment they have, and then they give you treasure or fall in love with you. Sweet, right?