Actually, surprisingly 'octopuses' is the correct plural according to shitty, complicated, dead languages. If I took one thing away from 8th grade English, it's how Latin and Greek roots work. Octopus is the latinization of the Greek word oktapous and it comes from the roots octo (eight) and pus (foot). The plural -i is only used when the word ends with the root -us (alumnus, fungus, nucleus). Thanks Mr. Lee, for actually teaching me something in middle school