I said oldest English plural. Octopi is the oldest plural in English for the English word “octopus.”

We took a word that sounded to us like a second declension Latin word and gave it a second declension plural. This wasn’t accurate in Latin, since it’s actually a third declension noun with weird Greek endings (as a word lifted from Greek).

But English doesn’t use declensions the same way Latin does. We just know that many words that end in -us get pluralized as -i in English (alumnus -> alumni, etc.) and so “octopus” as “octopi” sounds right to English-speaking ears.

Then some people were like, “Nah, it should follow English plural rules” and said “octopuses.” Then others were like, “Well, as a Latin word FROM a Greek word we should be using the proper third declension Greek ending plural from Latin” and we got to “octopodes,” which matches up with the Attic Greek masculine plural, «ὀκτώποδες» but pronounced differently because Latin didn’t differentiate the same way between Ο and Ω. And then we bastardize the pronunciation in English to blend the Latin and the Greek and our even further weakened English vowel to the point where we almost say “ah” for omega. (Which is why I wrote it that way.)

Anyway, the point is we shouldn’t be prescriptivist about the plural of the word octopus in English. Just let octopi and octopuses and octopodes live in peace with one another.

Octopus isn't a Latin derived word but Greek. You can't apply Latin grammar to Greek words.

There is no absolutely correct plural for octopus and in any respect, no grammatical rules should be prescriptivist (you must do this) but prescriptivist (people tend to do this)