Octopi, the supposed plural of octopus, is a favorite among fans of quirky words. But though it pains us to point this out, octopi is not etymologically correct. Octopus is of Greek, not Latin, origin, so its archaic plural would be octopodes, not octopi.
In any event, there’s no reason to use an archaic plural here. Octopus was derived from a Greek source, but it has been in English for centuries and is now an English word, so we can treat it as one. Countless long-established Greek- and Latin-derived words are pluralized according to modern English conventions, and there’s no reason why octopus should be an exception. Octopuses is fine.
Examples
People love octopi, so the quirky plural appears often—for example:
The former is served a martini glass, holding about a dozen mini octopi cooked in a sweet and spicy sauce … [Birmingham News]
A ban on fishing in the region may affect the market for octopi and marine-based food products popular in Los Angeles … [LAist]
Noise pollution knocks squid, octopi off balance [MSNBC]
But most major publications use the boring but perfectly acceptable plural, octopuses—for example:
Octopuses are highly intelligent animals and have been proven to have a strong short and long-term memory. [Telegraph]
Octopuses, turtles and rays glide along the seabed. [Sydney Morning Herald]
Cable had a great year, and media octopuses like Time Warner and News Corporation continue to find plenty of profits. [New York Times]
Octopodes appears occasionally, but it’s liable to cause confusion and is best avoided.
