Depends on the language. Current standardized American English overwhelmingly list adds -s/-es to make a plural noun. So โoctopusesโ would be technically correct with โoctopodesโ and โoctopiโ also accepted.
If youโre writing Latin, youโd want octopi instead. If youโre writing Greek: octopodes.
But English is a democratic language, so feel free to vote often and early by using your preferred version.