There are 3 plurals for the word octopus, as stated in the Oxford English Dictionary, which even the 8 legged mollusc might get confused with.
The first is the natural English pluralisation, octopuses. Simple.
The second, octopi, originates from the mistaken presumption that it’s a Latin word, and every public schoolboy who did elementary Latin will know that words ending with -us is pluralised to -i. However, it isn’t a Latin word, rather the Latin word for octopus is octopes, pl. octopedes.
In fact, it’s an ancient Greek word, from octo- meaning eight, and -pous meaning legs. Pluralise it in Greek and you get octopodes, the third accepted pluralisation of octopus in English.
But ultimately, most dictionaries will only offer octopuses as the only correct plural, indeed, according to Wikipedia, Fowler’s Modern English Usage states that "the only acceptable plural in English is octopuses," and that octopi is misconceived and octopodes pedantic.
Fine, call me a pedant. But better than being wrong!

Links:
Wikipedia: Octopus terminology
Tagged: , octopus, Random Facts
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One Comment
Aha! You’ve also managed to refine my understanding of the term "schoolboy error" in the same blog post. Well done!