If I recall correctly, Francis James Child's collection followed in the footsteps of the Danish ballad collector Grundvig, a personal friend of Child's, and one of his criteria in selecting ballads as genuinely "popular" was whether other arguable versions of the same ballad could be identified (say) in Ireland or Scandanavia. I also believe that the real pioneering ballad collections were German, identified with the Schlegels and the Grimms. Others doubtless know more about this, but I think that's a fair approximation.

Child says that parts of the ballad are identifiable in a 1629 Italian broadside. He writes:

"The corresponding German ballad has been known to the English for two generations through Jamieson's translation. The several versions, all from oral tradition of this century [the 1800's] show the same resemblances and differences as the English." He identifies one with snakes from Brandenburg, and then "Peter, 1, 187, No. 6, from Weidenau, Austrian Silesia, run thus: Henry tells his mother that he has been at his sweetheart's (but not a-hunting); he has had a speckled fish to eat, a part of which was given to the dog, . . . which burst. Henry wishes his father and mother all the blessings, and hell-pains to his sweetheart . . . [Etc.]" He goes on to observe that of several German variations, this seems one of the closest to the English, "and have even the name Henry, which we find in English C.

Child also identifies one Dutch, two Swedish, two Danish, two Magyar, and one or more Wendish (West Slavic) versions, and very similar Bohemian, Moravian, Slovak, Polish, Serbian, Bulgarian, Slovene, Russian, Massovian, Lithuanian, Albanian, Romaic [?] and Ruthenian versions. Generally, it seems the boy is served snakes or toads which are misrepresentified as fish.

Always this vulgar prejudice against toads!

The Child Ballads with annotations are available, very inexpensively, from Loomis Press. Malcom provides a clickie on this thread where it can be found.