Now, we probably don’t use either the singular or plural form of octopus very often unless we own or frequent sushi bars, but most of us have long been mistaken about how to pluralize the critter.
I’ve heard octopi used a lot, but that is a completely erroneous pluralization. The word comes from Greek, not Latin (Greek: oktopous, or eight-footed). Were it of Latin origin, octopi would be fine.
That leaves us English speakers with octopuses, which the dictionary confirms. However, the real, honest-to-God plural form is octopodes.
Bank that one in your memory if you ever plan on competing on “Jeopardy.” Otherwise, just go on saying octopi if you like.