Ah, yes. The tags, we got
squid, water, fish, hentai, funny
Wait, hentai?
Hentai it is then.
B-but wait i But he was already downloading 10 terabytes of hentai
That's possessive singular. And just 'octopus' is not an accepted plural form; those are octopi, octopuses, and octopodes.
Furthermore 'octopus' has its roots in the ancient Greek language, but the word as we use it today is derived from Latin, and nearly every word in the English language derived from Latin drops the -us and replaces it with -i for the plural form.
As is the case with the English language, the above rule has exceptions out the ass but that's the general rule of thumb to go by.
Only if you value ******** descriptive language. Octopi makes zero sense as a plural, since octopus is not a Latin-derived word. Octopuses is the correct English pluralization.
Merriam Webster, as an American company, has no business deciding these things.
As usual, the infinitly superior Oxford Dictionaries has us covered and says that Octopi is actually the only incorrect form as it arose due to a misunderstanding.