It’s been settled.
The plural of octopus is not octopi.
Octopus is from Greek, not from Latin. The Greek word is oktopous [ok.to.pus]. It was used in Greco-Latinate scientific language, so somewhere along the way people just thought it was all Latin and we should all speak it.
The plural in Greek is oktopodes [ok.to.po.des]. Who’s gonna use that?
Goes to show that digging in old dictionaries for foreign plurals as the “true” form is the work of wizards and alchemists—it’s chasing a fantasy. You can’t make it real in this world, as fun as it may be in the imaginary. Also goes to show how hypocritical and unsustainable it is to pretensiously hold on to archaic forms of language and require them of everyone around you or else you’ll debase and slander their intelligence.
Always nice to see Dictionary.com promote something of linguistic worth.