Although dictionaries will show words that significant parts of the population say are wrong, I’m still going to say octopuses is also correct because octopus is an English word and English usually pluralises like that

Octopusen is not common since octopus is too new to get that style of English plural

Of course significant parts of the population spell “than” with an e, or “then” with an a, and/or don’t know the difference between it’s and its

Would you care to pluralise octagon?