Grammar Girl:
We love Christmas carols in the Grammar Girl household. This year, the song "I Want a Hippopotamus for Christmas" got me thinking about the plural of "hippopotamus."
"Hippopotamus" comes to English through Latin from the Greek word "hippopotamos," which means "river horse." You make some Latin nouns plural by changing the ending to "i" (e.g., "dominus" becomes "domini" in Latin and "fungus" most commonly becomes "fungi" in English), but this is not true for Greek nouns. Further, Latin nouns that become common in English often become naturalized and take "s" or "es" instead of "i" to become plural.
Since "hippopotamus" comes from Greek and is a relatively common word in English, the standard plural is "hippopotamuses," although some dictionaries do list "hippopotami" as an alternative.
