When children are learning the idiosyncrasies of the English language, they can get tripped up by some of the different ways singular nouns become plural. This is especially true when they’re learning about the animal world. Why don’t we add an S to make deer and sheep plural? If it’s one goose and two geese, why isn’t it one moose and two meese? When does the plural of fish stay fish, and when is it fishes?
By the time we become adults, we’ve worked most of these out, though there are still a few obstacles that could land us on our metaphorical faces. But they needn’t be a problem. As you’ll see from the three examples and conclusion below, pluralizing these words doesn’t have to be that hard. Finding the right plural becomes a problem only when you think about it too hard or too much and wind up over-correcting.
There’s a long-standing conception that the plural of octopus is octopi, based on other similar-sounding Latin-based words like alumnus/alumni, cactus/cacti, and fungus/fungi. But octopus isn’t Latin. It came from Greek oktō- “eight” and -pous “feet.” Therefore, to be hyper-correct, the word ought to use the “correct” Greek plural ending, right?
By the same token, some advocate that the plural of platypus, from the Greek platy- “flat or broad” and -pous, ought to be platypodes. There’s an argument to be made there, but not a very good one, because this word didn’t come directly from Greek. It was created from the New Latin that developed during the Renaissance and which sometimes borrowed from Greek. So the Latin plural platypi is arguably as correct as platypodes.
Even less common is the appearance of the Greek plural ending for the word rhinoceros, from the Greek rhino- “nose” and –keros “horned,” traveling through Late Latin into Middle English. Though you can find many uses of the Greek plural ending in the sometimes hoity-toity texts of the eighteenth and nineteenth centuries, today, the hyper-correct rhinocerotes is more or less reserved for specialist scientific papers and for the hyper-pretentious.
The debate over which plural ending to use — Latin or Greek — shouldn’t be as controversial as some (including, perhaps, me here and now) make it out to be. Why? Because when a word is adopted into the English language, it becomes an English word, and therefore, with relatively few exceptions, abides by standard English morphology.
And that’s usually the safe way to go. For accuracy, readability, and reader comprehension, it’s hard to go wrong with octopuses, platypuses, and rhinoceroses.
Nonetheless, you’ll still find octopi, platypi, and possibly rhinocerotes listed in your dictionary — not because lexicographers approve of them, but because modern dictionaries are descriptive, not prescriptive. Dictionaries don’t tell us which words to use; common usage tells the dictionaries which words and word forms to include. It’s up to you to choose.
But generally, choosing the standard English pluralization will rarely lead you wrong.