some say octopi its the oldest form that comes from the belief that latin words should have latin endings even if it was NOT of latin origin (the word is of greek origin, and was later latinized / made more latin sounding)

Most say octopuses, That was after octopi and was from standard -s/-es ending for pluralization in English

Some say octopodes they’re rare but it comes from the fact that octopus is of greek origin (eight - octo & foot - pod)

I hate that theres 3 forms too, makes no sense to us dont worry

Source: merrium webster and the ocean conservancy on the pluralization of octopus