English is full of irregular plural forms based on Latin and Greek. They can be confusing (apparatus? apparati? apparatuses?). They can be fun (the brothers Winklevii! and the flying Elvii! all cleaning their Prii! with Kleenices!). And they can be weapons of petty pedantry ("um, I think you meant to say the data are interesting…").
Here are some uncommon but etymologically sound plurals that you may employ for petty pedantry at your own risk. You are better off using them in the fun way, though they are most likely to be received as confusing.
There is something about the normal plural "octopuses" that just feels wrong, even though it is correct English. This why those concerned with giving the word the proper weighty scientific ring usually turn to octopi. If you want to one-up the octopi people, point out that the pus in octopus comes from the Greek pous for foot, and not the Latin second declension masculine ending, making octopodes the correct form. They will surely love you for it. (You may also use this strategy for platypodes.)
Does the reasonable "rhinoceroses" sound unreasonable to you? You might be tempted toward rhinoceroi or rhinoceri, but the ancient Greek pedigree of the word commands us to use rhinocerotes, the use of which was considered annoyingly show-offy even in the classic-crazy 1800s. So… it's perfect!
This Greek plural form for climax is usually found in discussions that also use the Greek meaning of the word—ladder. But it did get a bit of use in the 1800s in fancy writing about literary, musical, and dramatic climaces and anti-climaces. It was also used in the 1940s and 50s Kinsey reports on sexual behavior in reference to "multiple climaces."
You may not have many occasions to talk about more than one chrysalis, but keep the word chrysalides in your pocket for when you do. Also, amaryllides.
You probably have even less of a chance to speak of more than one Cyclops, so you will have to create the occasion yourself. "You should be a Cyclops for Halloween too! We'll make the cutest pair of Cyclopes!"
You may know that in the plural, stigma becomes stigmata and schema becomes schemata, but this pattern also properly applies in the making of plurals that almost no one ever uses. Why say enemas when the rules of classical Greek pluralizing let you say enemata instead? It's so much classier! Also, enigmata, aromata, glaucomata, and miasmata.
Onus is already a pretty fancy Latin word for a burden or obligation. The plural is onera. If your mom says the onus is on you to keep your grades up and keep your room clean, tell her you will accept these onera for a slightly bigger allowance. She will be so impressed that she just might give it to you.
If you visit a football stadium and a baseball stadium, you have visited two stadia. Sports fans are very impressed when you tell them about all the stadia you have visited.
There's a moment at the beginning of every college course where the professor must decide whether to pass out the syllabuses or the syllabi. You can help by pointing out that according to the Oxford English Dictionary the word syllabus is derived from a misreading of the Greek sittyba, so the plural should actually be sittybae. Then help pass around the sittybae, confident in your new role as the most popular kid in the class. Right?
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