[suzakus]

Yes, octopi is wrong in latin. Octopus is conjugated as Octopodi in the plural. See: https://en.wiktionary.org/wiki/octopus#Latin

[fishtockos]

See my sibling comment - where does the insistence that octopus is third declension come from? The reality is that while most Greek loanwords are third declension, many are not, and Latin authors tended to not be consistent.

[remarkEon]

Totally, it would still be wrong for the reason you cited.

Wasn't my point, however. I'm really just being stubborn on the plural actually being Octopodes :)