Of the three words commonly cited as a plural for octopus, octopuses is the preferred term. The others are octopi and octopodes (pronounced ok-TOP-ɘ-deez, or ɒk’tɒpədiːz in the International Phonetic Alphabet). Fowler’s third edition claims that the only acceptable plural is octopuses, and that octopi is misconceived. Fowler’s second edition is still more blunt, calling it wrong. Perhaps, but octopi has been used by many reputable publications and raises fewer eyebrows than octopodes. The popularity of octopi, however, seems to be waning on both sides of the Atlantic.
Fowler is not alone in rejecting octopi; some linguists do so on the grounds that octopus was not originally a Latin word but a Greek one – hence the pedantic plural octopodes, which is rarely if ever seen outside dictionaries, usage guides, and blog posts such as the one you are reading. Merriam-Webster’s Dictionary of English Usage counters that octopus was imported not from Greek but from New Latin, which took it from the Greek oktopous. This gives octopuses a grammatical edge over octopi, as well as historical precedence (original citations in 1884 and 1922 respectively).
If you want to use octopus in the plural, choosing octopuses should forestall accusations of inaccuracy, irregularity or obscurantism. And if you want a break from etymology, here’s an octopus being amazing. Some commentators have described its remarkable communication strategy as a way of wearing its language on its skin.