Apparently people have been arguing about this for at least 140 years.

The confusion arises because English just throws bits of other languages into a blender. “Octopus” came to English from the original Greek via a stay in Latin, and all three languages use different plural forms.

“Octopi” (Latin) is the most wrong of the available options, and “octopodes” (Greek), whilst technically sort of correct, sounds pretentious. We use “octopuses” (English), the most commonly-used form, but we won’t argue if you want to use something else.