From https://en.wiktionary.org/wiki/octopus#Usage_notes -
> Usage notes
> The plural octopi is hypercorrect, coming from the mistaken notion that the -us in octōpūs is a Latin second declension ending. The word is actually treated as a third declension noun in Latin. The plural octopodes (Latin: octōpodēs) follows the Ancient Greek plural, ὀκτώποδες (oktṓpodes). The plural octopii is based on an incorrect attempt to pluralise the word based on an incorrect assumption of its origin, and is rare and widely considered to be nonstandard.
> Sources differ on which plurals are acceptable: Fowler's Modern English Usage asserts that “the only acceptable plural in English is octopuses”, while Merriam-Webster and other dictionaries accept octopi as a plural form. The Oxford English Dictionary lists octopuses, octopi, and octopodes (the order reflecting decreasing frequency of use), stating that the last form is rare. The online Oxford dictionary states that the standard plural is octopuses, that octopodes is still occasionally used, and that octopi is incorrect.
> The term octopod (both octopods and octopodes can be found as the plural) is taken from the taxonomic order Octopoda but has no classical equivalent, and is not necessarily synonymous (it can encompass any member of that order). The uncountable use of octopus is usually reserved for octopus flesh consumed for food ("He ate too much octopus last night.").