The OED says that 'platypus' is modern Latin, though adapted from Greek 'platupous'. As it's Latin , all pedants may use platypi for the plural [we take it as a masculine noun of the second declension] and everyone else can laugh ! Platypuses seems better.
The dictionary gives 'platypussary' for "an enclosure or building in which platypuses are kept". The dictionary is there using 'platypuses' as the plural.