A platypode would be the correct pluralization if the word was from Greek origins. And it is. So technically Platypode should be the correct form of pluralization.
But its not. English is a screwed up language.
Much like octopoodes, the English language simply steals words and then destroys them into horrible bastardizations and now we have octopi.
Yeah, like octopuses. It is no platypi.
Platypi. As it lets you show off your Latin.