In Michael Ende's Momo, the tortoise Cassiopeia seems to be something of a mystery.
Even Professor Hora is easier to grasp: he appears to be a sort of personification of time itself, sending everyone's own allotment of time to them, with the flow of time all across the world stopping when he sleeps. What he gives, the men in grey poison and steal; they're essentially time parasites.
But Cassiopeia? She doesn't seem to serve any vital role in Professor Hora's work or the normal operation of time, mostly acting as an advisor, confidante, and helper to Momo. Her power of seeing half an hour into the future is one unmatched (as far as I remember) by any other person or creature in the story. She's the only thing able to move during the time-stop, apart from those with external sources of time like Momo's flower or the men in grey's cigars.
What manner of being is she, really? Why are so many unique abilities found in a single tortoise? Is she something else taking the form of a tortoise - and why a tortoise specifically? Are there more like her anywhere in the universe?