Octopuses vs. octopi

Image from: The Living World of Molluscs, http://Molluscs.at

Thank you to a recent comment from a reader questioning whether the proper plural form of octopus is octopuses or octopi. Here is the "official" answer from the editor at Merriam Webster:

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Ahaha, awesome. One of my TAs in the past had a huge crusade going about getting people to say octopuses instead of octopi. It got to the point where we automatically corrected our professor when she said octopi, The TA was very proud. Though I'm tempted to say octopods just to be pretentious now.
In any case, Google Chrome's spell check seems to agree with "octopuses" and not "octopi," and we all know that's the final word.

Can I just keep saying 'octopussies'?

I read "octopussies" in a Discworld novel. I think it's a lovely word and use it at the slightest provocation.

We all know octipodies (or however you spell it) is the original Greek ending and thus the correct one - the other two have become valid by common usage. What I want to know is, what is the proper plural form for penis? Penises? Peni? Penepodies? Penelopes? Panoplies? Not that I've had any personal experience with the plural penis, but for purely theoretical reasons I'd like to know.