Some English teachers insist that the correct plural of octopus is octopi and remove marks from student's work if they use the spelling octopuses. The argument goes that since the word octopus comes from the Latin language it should use a Latin plural. English teachers who love Latin so much should take a Latin course. If they did they would learn that the Latin word for octopus is polypus, not octopus.
So where did the word octopus come from? It originally comes from the Greek language and means eight-footed. The Latin word for the same animal, polypus, means many-footed. The Greek plural is octopodes. A few clever English teachers use this Greek plural in English just to make the point that the spelling octopi is wrong. What they are really pointing out is that they are cleverer than their old English teacher who took marks off for using the spelling octopuses.
Some dictionaries and English textbooks claim that octopuses is the only acceptable plural in English. They are generally correct; however so many people use the erroneous plural form octopi, that some other dictionaries will accept it as an alternative.
In English it is normal to borrow words from other languages. The normal plural is created by adding "s" or "es" no matter what was done in the original language. So in your own writing it is best to use octopuses. If your teacher wants to remove marks for not using octopi get an English dictionary and a Latin dictionary and show them the entry for octopus. If you want to use octopodes you should be prepared to explain why every time.