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platypuses or platypi are both correct.
Plural of "a platypus" is "platypuses" or... "platypus". This second form is promoted by The Australian Platypus Conservancy. "Platypi" is an error, because the change "us -> i" should be applied only to words of Latin origin while "platypus" is of Greek origin.
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A group of platypus is a platyplot.
A good one!

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A baby platypus is a puggle.
Not exactly. "A puggle" is a baby echidna. There is no common term for a baby platypus, yet "a platypup" is suggested.
For the language nuances look here:
http://platypus.asn.au/historical_background.htmlNow about that mysterious tenth platypus from the first post. I guess that the tenth platypus knows you have no money for an ounce of platinum.