I suppose it could, Stefan... but any fish would have the same problem as a sea-slug: take it out of water and it would quickly deflate.

Besides, I thought of something else: the author of that article Mudguard posted mentioned that the prince who succeeds is #100 -- the number of completeness. Well, the first place he hides is in a Raven's egg (air), the second place he hides is inside a fish (water)... to complete the triad he'd need to hide in some way that represents earth... so a guinea pig/ would fit that better than another kind of fish...