Could it be that it just don't make *sense* asking what *real* existing animal is meant by 'Meerhaeschen' - just like asking which *real existing* (human?) being could be meant by 'Rumpelstielzchen' (Rumplestilzkin - by the way another diminutive: '-chen') Why not leave it to (children's) fantasy to *imagine* what might be a 'Meerhaeschen' so tiny that it can hide in the hair of somebody)? Couldn't it be the Grimm's *motivation* to pick up a name which they themselves may have heard somewhere (i.e. from the 28. chapter of Jean Paul's 'Flegeljahre' who himself takes the name for a heading, but doesn't relate to it anywhere in his text, perhaps out of the same motivation!?), but didn't even know the exact animal behind the word (or - if they knew - could imagine that readers/listening children couldn't know about the exact meaning, may it be a snail or a fish or anything). In shorter words: 'Meerhaeschen' only being used as a means to stimulate the reader's /listener's imagination !?!