Plurals for hippopotamus and octopus


It is a popular and well-known fact that the correct plural for “octopus” is not “octopi,” but “octopuses” or “octopodes.” This is because the word “octopus” is Greek in origin, not Latin, and the Greek word for the word “pos” (foot) is “podes.”

Here’s my question: If it is incorrect to pluralise “octopus” as “octopi,” then why is it okay to pluralise “hippopotamus” as “hippopotami?” The OED says that this is an acceptable plural.

The word “hippopotamus” is also Greek in origin. It comes from “hippos ho potamios.” In Greek, “hippos” means “horse,” and “potamos” means “river.” The plural of “hippos” is “hippoi,” as I recall.

So shouldn’t the plural be “hippoipotamus” or maybe “hippopotamoi?”


    title = {Plurals for hippopotamus and octopus},
    journaltitle = {The Grey Literature},
    author = {Benjamin Gregory Carlisle},
    address = {Montreal, Canada},
    date = 2011-05-13,
    url = {}


Carlisle, Benjamin Gregory. "Plurals for hippopotamus and octopus" Web blog post. The Grey Literature. 13 May 2011. Web. 16 Dec 2018. <>


Carlisle, Benjamin Gregory. (2011, May 13). Plurals for hippopotamus and octopus [Web log post]. Retrieved from

8 responses to “Plurals for hippopotamus and octopus”

  1. Matthew Dugandzic says:

    I have been asking myself the same question for a while now. I think that “hippopotamoi” would be “horse of the rivers”. I’d suggest “hippoipotamus”. Or maybe “hioppoipotamoi”.

    Je me pose la même question pendant beaucoup de temps. Je pense que “hippopotamoi” serait “cheval des fleuves”. Je suggerais “hippoipotamus”. Ou peut-être “hioppoipotamoi”.

  2. Matthew Dugandzic says:

    Hmph. I just plugged “hippopotamus” into Google Translate and it said “Ιπποπόταμος”. Then I did the same thing for “hippopotamuses” and it said “Ιπποπόταμοι”. So, there you have it, in Greek.

    Hmph. Je viens de mettre “hippopotame” dans Google Translate et il a dit “Ιπποπόταμος”. Puis, j’ai fait la même chose pour “hippopotames” et il a dit “Ιπποπόταμοι”. Voilà, en grec.

  3. Murph E. says:

    Well there you go!

    I wonder if it would take the internal pluralisation if you were literally saying “horses of the river”—”hippoi ho potamios” ?

  4. Matthew Dugandzic says:

    Well, “the horses of the river” in Greek would be “oi ippoi apo tou potamou”. Literally, “The horses from the river”. I guess “hippopotamus” is a compound word that is pluralized as its own entity.

  5. Benjamin Rossen says:

    ‘Hippoipotamus’ is strictly correct, I believe, but since many Greek words were incorporated into Latin in Ancient times, words with Greek roots can also be used with Latin inflections. The same can be said for ‘hippocampus’, of which there are two in the brain, so these should perhaps be known as the ‘hippoicampus’ but here too one sees ‘hippocampi’ written.

  6. Jacqui says:

    I’ve been thinking about this for a few minutes now and I believe Greek or not it’s hippos this solves all problems

  7. Chris Davis says:

    I have been saying that it is the horses that are plural, not the river. So hippoipotamus it should be.
    Eric Partridge argued that the plural of lion was lion. The ‘s’ is incorrect.

  8. Keith Irwin says:

    I had had this same question for a long time. People would tell me that the reason that octopi wasn’t a proper plural was that we had gotten the word octopus from Greek. But this, of course, wasn’t true. If we had, it would almost certainly be octopous, which is essentially how it is spelled in Greek. We got it from Latin which got it from Greek. So I always wondered why we didn’t apply the same standard to hippopotamus which is also a word we got from Latin which got it from Greek. It turns out that the real answer is this: in Latin, the correct plural of octopus is not octopi. Not all nouns ending in -us become -i in the plural. Hippopotamus does, but octopus doesn’t. So what is the proper Latin plural of octopus? Well, in Latin, it’s octopodes, which, of course, they took from the Greek. So that’s why octopi is incorrect. It’s not the Latin plural. It’s just a bad guess at the Latin plural.

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