The Latin (nominative) plural is octōpodēs, the Greek (nominative non-neuter) plural is ὀκτώποδες. Almost identical. octopi is not used in Latin, it only appears to occur in English texts.
Most Romance languages get their primary word for "octopus" from an alternative Latin word for it, polypus (plural polypī), which comes from Greek πολύπους (plural πολύποδα). polypus is also an alternative word for octopus in English, albeit it is rather archaic. The English plural polypi cannot be accused of being a hypercorrection, but (for octopuses) polypuses is much more common. polypus is also medical terminology for a type of blood clot, and the polypi plural is often preferred when the word is used in that medical sense.
The usual Dutch word for "octopus", octopus, is pluralised by the standard Dutch rules, octopussen. English appears to be the only language in which this hypercorrect pseudo-Latin plural has any popularity.
Snicker ... The new Webster or Oxfords ...
Though, in either case, it has the danger of suggesting the plural form of the hypothetical word “processee” (from processor, and by analogy to pairs like lessor/lessee), so, “things that are processed”.
..since when? I don't think I've ever heard "processees" before...
Octopi is not the most common plural, true.
> Octopus is of greek origin, not latin.
Octopus entered English from scientific Latin. In which the Greek plural was not used. (Lots of words in English fron languages other than their language of origin and have irregular English forms from their immediate source rather than their remote origin.)
“Octopi” appears to be the earliest English plural of octopus.
> Thus, its greek plural is octopodes.
“Thus” is incorrect there, but, yes, that is its Greek plural.
> And thus, since we don't tend to use greek plurals in english
But we do. Just not, until comparatively recently and never dominantly, in the case of “octopodes”.
> the other proper plural is octopuses.
That is the most common English plural, and you don't really need a (false) historical narrative to support that it is “proper” except coming from a broken prescriptivist framework.
> Octopi is what's known as a hypercorrection
Since its the oldest attested English plural, its not a hypercorrection, just (arguably) dated. Though its probably the most common incorrect example of hypercorrection.
https://www.merriam-webster.com/words-at-play/the-many-plura...