What’s the plural form of the word ‘octopus?’ Many would respond with “Ah, I know! It’s not octopuses… it’s octopi, right?!”
Not really, no.
Octopus is formed from the Greek words ὀκτώ ‘okto’ eight and πούς ‘pous’ foot. In accordance with Greek rules, it should be pluralized as octopodes.
Some people assume incorrectly that it is a Latin word (like stimulus or cactus) and should therefore take the Latin 2nd declension plural octopi (analogous to stimuli and cacti). Since this is based off a bad assumption, most prescriptive dictionaries do not accept ‘octopi’ as a valid form.
According to English rules, it should pluralize to octopuses, which is equally acceptable as octopodes according to prescriptivists. Octopuses is more common than octopodes in actual English usage.
You can verify the above this using the Online Etymology Dictionary (or any reputable source, for that matter).
Image credit: http://marthamouse.files.wordpress.com/2011/08/octopus-group-photo.jpg