MudGuard: states that "Meerhäschen" in some cases has the meaning of "Kaninchen" (rabbit), but in this case it should be an animal with the latin name "Apiysia kurodai" (whatever this is - I couldn't find anything on that)
Jeanie: Aplysia kurodai is a sea slug!
Looking at the photos, I suppose their various protuberances could look a bit like hare's ears, hence the name.
I wouldn't mind having one for a pet, anyway...they don't look as though they would be much trouble !
Neither would I... in a a fish tank. I don't think I'd feel much need to carry in my arms as I look out on my kinggdom for an entire day, or to let it crawl up on my shoulder, right against my neck, though (which is what the princess in this story does). And I don't think a sea slug would be happy with that arrangement, either.
.... I just read an altavista translation of the Web page Mudguard posted, and while the author makes the argument that a sea slug does make sense, because it is smaller (i.e. easier to hide) and more exotic than an ordinary rabbit, it still comes across as rather odd, especially since the transformation takes place in a mountain forest. So I'll just stick to "rabbit"
This story reminds me of another I've read from a collection of Scandinavian tales. I don't have the collection within reach at the moment, but I think the story is from Finland...
In this story, a young man also wishes to marry a princess, and she agrees to -- if he can hide from her while she looks in a magic mirror (? I think -- I'll have to go back and reread it). An old wise man/mentor wishes to help the young man, and so gives him a spell that opens up the hearts of creatures so he can hide inside. First, he hides in the heart of a bear, but the princess recognizes that the bear is acting differently, so she calls him out. So next, he hides in the heart of a rabbit, but again, she finds him. At last, the young man opens the heart of the princess and hides there, and because she herself has changed, she doesn't recognize it, and so admits defeat. Then the young man comes out and says: "I was here all along."
A much more romantic ending, but the story leading up to that point is rather weak, so I'm using the Grimms' story instead.
Anyway, there is a European cousin of the story where the rabbit shows up, even if it's not the final answer.
And, hey! Thanks guys!