You're missing a word, but it seems like you're saying octopuses is wrong. But it's right, and not just because a spot informal weird had been accepted: it's technically correct. As sibling comments have said, the original plural would be octopodes because it's from the Greek, not octopi because it's not from Latin, but English doesn't normal take Greek plurals.

Apparently the majority of English speakers say octopuses (which is my preference as well, but I'm not a native speaker).

By the way, how do you pronounce octopi? Is it "octo-pee" or "octo-pie"?

pie