I don't know why so many people try to be picky about the plural and then repeat this same error. The Greek word for foot is πούς. The word octopus was derived from this, but like all scientific names is formally Latin, both in spelling and how people declined it.
It just so happens the Latin plural is octopodes, because it's not a declension that goes from -us to -i . And yes, this is based on the Greek roots, but that doesn't always happen; the plural of the similarly-derived polypus ended up polypi, and now saying polypodes would be incorrect.
And of course that's if you want to ignore that now we're using the English word octopus, with whatever plural English people care to give it. We did use octopodes in the past, but if now people want to use octopi, it's kind of silly but no different from how the Romans made up polypi.
I'm not sure what the point there is in being annoyingly pedantic and not bothering to get things right. We can be better than insisting Latin names aren't Latin, surely? Please?