I love Christmas music in general, and one song that I think is especially cute goes like this:
I want a hippopotamus for Christmas.
Only a hippopotamus will do.
But being Grammar Girl, the song also gets me thinking about the plural of hippopotamus because it’s a common question: Is the plural hippopotamuses or hippopotami?
'Hippopotamuses' or 'Hippopotami'?
Dictionaries list both plurals, and both forms regularly appear in writing that you’ll see today.
The word originally comes from Greek where it meant “river horse,” and it came into English through Latin.
Hippopotamuses is the best choice.
Hippopotami is the correct Latin plural form and still regularly appears in scientific writing, but words that come into English can also shed their Latin plurals and take the more Anglicized -s or -es form, and hippopotamuses appears more often in books and newspapers.
For example, a Google Books Ngram search shows that hippopotamuses is slightly more common than hippopotami, and a New York Times Chronicle search shows that the Times clearly favors hippopotamuses—hippopotami hasn’t appeared in that paper since the 1930s. Garner’s Modern American Usage also recommends hippopotamuses.
And hippopotamuses like me too
She was using the correct plural.
The New York Times also occasionally uses hippos, and although hippos sounds a little more informal, it’s a nice plural because it avoids the whole problem of which plural to use.