'Octopi', interestingly enough, is a result of people assuming that the word 'octopus' comes from Latin because it ends in -us, which would mean that the correct plural drops the 'us' and replaces it with an 'i'. But, trickily enough, it does not come from Latin; it comes from Greek. As a result, if you're trying to be super technical, the correct plural would instead replace the 'us' with 'odes'--octopodes.

Of course, almost nobody actually uses that term unless they're doing it for fun. The most commonly used, correct plurals for octopus are 'octopuses' and 'octopus'.

I hope you enjoy that little tidbit as much as I did when I learned it.