Earlier quoted context omitted.
It may be a word like "fishes". I once saw "fishes" on an interpretive sign at an aquarium and some folks were mocking such and obvious grammatical error. Turns out, "fish" is the correct way to refer to multiple individual fish as a group, whereas "fishes" is the correct way to refer to multiple species of fish. I wonder if "octopi" might refer to multiple octopus without making any indication as to weather they are…
There’s no right way. Octopus is a word created in modern scientific latin by nonnative (scientific) speakers out of greek parts brought into English. There are no real rules there. It is not a loanword from Greek, it was meant to be a scientific latin origin word, but not native latin like other Latin words that have -us as endings for nouns. It’s a mess, there’s no answer, pluralize as you like but don’t go telling…
I recall reading that back in the day, there were criticisms that the neologism "television" would never catch on ... because it combined Greek and Latin roots.