Words in English that end in “-us” and pluralize to “-i” are from Latin, such as cactus/cacti or focus/foci. However, “octopus” is from Greek, and its correct pluralization from Greek would be “octopodes.” In English, we have also adopted “octopuses,” which fits our standard pluralization rules.
So, “octopi” is actually a language mix up, though one that’s so common that it seems to be accepted in English.
IMO it’s tragic to eat such an intelligent creature. Maybe stick to these guys…? Mmmm.