Being neither an expert at hares nor at fairytales I suggest the following solution: 'Meerhase' or 'Seehase' (Cyclopterus lumpus) is a north atlantic fish, whose (black) eggs are used as a caviar ersatz in Germany ('Deutscher Kaviar'). Such an egg (a 'Meerhaeschen'?) could easily hide itself (remember: it's fairytale!) in the hair of a princess - couldn't it ?!?