Elf-maid

On the other hand, the kobolds, brownies, and other household sprites who are not merely benign, but helpful, if well treated; the elf-maids who marry mortals, the swan-maidens, and the like, bear the stamp of Teutonic fancy.

If Atle had believed in trolds and elf-maids, he should surely have supposed that Lars must have seen something of the kind on his night walk in the forest.