An excellent question! The original Greek word was “oktōpous” which was latinized/modernized to “octopus.“ Because of this Greek root, the original plural is “octopodes” (oktṓpodes). However, the most common plural form in English is “octopuses,” which is what we use here at the Aquarium.
The plural “octopi” comes from the incorrect assumption that “octopus” is a Latin word. But, it’s so popular that it’s in the dictionary, and even though it’s not technically correct, it still gets the point across that you’re potentially dealing with several octopuses, so go for it if that’s what works for you.
Tl;dr: “Octopuses” is the plural we use here at the Aquarium, “octopodes” if you’re texting Poseidon, and “octopi” also works, especially when ordering eight desserts.