Folksong? Well, perhaps if you are referring to the little nursery rhyme
Lavender's blue, dilly, dilly,
Lavender's green;
When I am king, dilly, dilly,
You shall be queen.
(and maybe a few more innocuous verses, variously added, and sung to defenseless children by parents and relatives).

The Songs reproduced above have been printed in a few books (Roxburghe, etc.), but are known mostly from old broadsides, the words strung together by some alcoholic versifier trying to get enough jack to buy his next bottle. The writers who wrote the books seldom gave a source (read the posts by Bruce O, above). The one collected by Linscott in New England similarly lacks documentation.

Baring-Gould took a few words sung by some woman and put together a full song, using a book. No telling where the woman had heard, or more likely read, the original.

People (like me) who like old songs- they tell us a lot about what the long dead believed and how they lived- dig them up.