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.