Octopus, from the greek "octo" and "pod", meaning "eight" and "feet, respectively, was a latinized Greek word. In being an English word, "octopuses" was made the 'correct' plural. However, due to its seemingly Latin spelling, scholars changed its plural to "octopi", with rational similar to the pluralization of "alumnus" which would be "alumni".
That was completely wrong.
Octopus is a latinized Greek word, and therefore should be pluralized in the Greek form, akin to the "alumnus" pluralized in a Latin manner. Hence, following Greek pluralization rules, the plural of "octopus" should be "octopodes", which is pronounced "ahk tah poh dees". However, "octopuses" is valid, because it has been adopted into English, where the pluralization is sometimes modified for consistency.