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I've seen "octopuses" and "platypuses", respectively, but I've also seen "octopi" and"platypi". Which is correct, and why?

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Octopuses and platypuses would be correct. Octopi and platypi masquerade as Latin plurals on the false premise that the names are masculine Latin nouns of the second declension, and they are no such thing. The -us at the end is not analogous to the -us in fungus; rather it is part of the suffix -pus which is a Latinization of Greek ποῦς, foot. If you want to pluralize it in the true manner of its classical root, you would have to go with -podes. And that would be, frankly, a ridiculous extreme of pedantry.

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    Octopi is listed in various English dictionaries as a valid alternative. I wouldn't choose it myself, but this is perilously close to the etymological fallacy. (Platypi is also given by RHK Webster's.)
    – Edwin Ashworth
    Apr 2, 2015 at 23:15
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    On the other hand, the Latin word for octopus is polypus, and the plural is polypi. It's not that wrong.
    – Peter Shor
    Apr 2, 2015 at 23:19
  • And then you will then have octopodidae as the family which includes the octopuses. In French I see you can also form plurals from the singular -pode to -podidés as in octopodidés(fr) i.e. a familiy included in the octopodes(fr) order which includes the octopus.
    – user98955
    Apr 2, 2015 at 23:36
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    Ridiculous extreme of pedantry? Ouch! Octopodes and platypodes are hardly etymologically defendable (as comments in the question this one is marked as a duplicate of show), but it’s what I personally grew up saying, and it's the normal plural for me. I realise I’m in the minority here (and octopuses/platypuses sound perfectly fine to me as well—they’re just not what I’d normally say myself), but I don't think I’m that pedantic …
    – Janus Bahs Jacquet
    Apr 3, 2015 at 0:02
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Platypuses and octopuses are technically correct because both are not latin words although either is fine. On the other hand, there are some English words that do have correct plurals that end with -i such as hippopotami (hippopotimus) and radii (radius).

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    What does 'technically correct' mean? It sounds like 'are formed according to traditionally more acceptable morphological rules'. But usage is what ultimately governs acceptability; recognition of false appeals to etymology for acceptability has actually encouraged the coining of the compound noun 'etymological fallacy'.
    – Edwin Ashworth
    Apr 2, 2015 at 23:35

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