The plural 'octopi' is based on a grammar mistake — 'octopus' is Greek, not Latin.

The “-i” plural ending, as in “cacti” or “fungi,” applies to Latin words ending in “-us.” “Octopus,” however, comes from Greek, combining “okto,” meaning eight, with “pous,” meaning foot. Applying a Latin plural rule to a Greek-derived word is a mismatch, even though “octopi” has become common enough in everyday English that many dictionaries now list it as an accepted variant.

The more etymologically consistent Greek plural would be “octopodes,” though it’s rarely used outside of pedantic corrections, since it sounds unfamiliar to most English speakers. In practice, “octopuses,” formed with the standard English plural ending, is the form most style guides and dictionaries recommend as both correct and natural, sidestepping the Latin-versus-Greek argument entirely.