Let me try to solve the mystery of more than one platypus:
Platypi (Latin) also is the modern pronunciation of platypoi (Greek). Both forms are wrong, unfortunately. Like with the octopus (eightfoot) one part of the name platypus (flatfoot) is Greek pous (foot), stem: pod*. So the correct plural should be platypodes.
But for the average use I think we can stick to the homely platypuses. But be aware of a clear and distinguished pronounciation: in slurred speech it could be mistaken for platypussy (the platypus' girl friend).

A hail to Fosters and McGrath for the marsupials around Loch Lomond!
Wilfried