For you geeky linguists out there, octopi is not the plural of octopus. Some idiot thought it followed the Latin -us/-i scheme. But it’s a Greek word — so the classical plural is octopodes.
... and not only that, it’s not a classical greek word. I guess they weren’t so concerned about the exact number of tentacles, because they called it the polypous - it’s in Homer, The Odyssey Book 5.432, where it’s written poulypodos. ( A very interesting passage, actually. )
( Any knowledge of greek I may have comes from poking through Homer. )