The more usual Greek word seems to have been polypous (also pōlyps), from polys “many” + pous, but for this word Thompson suggests folk-etymology and a non-Hellenic origin.

The classically correct Greek plural (had the word been used in this sense in ancient Greek) would be octopodes. Octopi regards the -us in this word as the Latin noun ending that takes -i in plural. Like many modern scientific names of creatures, it was formed in Modern Latin from Greek elements, so it might be allowed to partake of Latin grammar in forming the plural. But it probably is best to let such words follow the grammar of the language that uses them, and octopuses probably works best in English (unless one wishes also to sanction diplodoci for the dinosaurs).