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.
Octopodes is correct if you want a classical (Greek, as it is) word. Octopi is an assumption of correctness by analogy of people who don't know better, like 'if you have any feedback give it to my colleague and I' - it's not all posh and correct to say 'and I', it's wrong (in that sentence).
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.