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 anyone there’s a right way because there isn’t. It’s a greek, latin, and english word, but also none of them. No usage is standard or ultimately correct.