What is the plural of octopus? Is it octopi or octopusses?
We would go with "octopuses," a perfectly legitimate English plural, and the oldest attested to. "Octopi" is also an acceptable choice, and one in wide use, but you run the risk of being informed that it's incorrect. Well-meaning people may tell you that -i is a Latin plural, but "octopus" comes from the Greek. While this is true, the word in fact went from Greek to Latin, and only then to English. While the plural is irregular, it's hardly out of bounds.
Those who don't like "octopi" sometimes suggest that the correct term is "octopodes." Nobody ever uses "octopodes" outside of arguments about the plural of "octopus." Nor should you.
In short, stick with "octopuses" unless you really prefer the sound of "octopi."
—The Fact Monster