Fun fact:

All words and grammar rules are made up. Entirely. As in... invented by humans. The laws of physics don't really care.

If lots of people use "octopi" (and they do) and most people understand what it means (which they do) then congrats! It is, in fact, a word. If enough people apply an "i" ending to words then that becomes itself a new grammar rule.

English, just like every other language, also has a ton of unwritten grammar rules as well as spoken word only rules.

In short: octopi is in fact a perfectly cromulent word.