Plural of Hippopotamus

What Is the Plural of Hippopotamus?

The Quick Answer

The plural of "hippopotamus" is "hippopotamuses" or "hippopotami."

Hippopotamuses live in groups of up to thirty.
Though they are bulky animals, hippopotami can gallop at 19 mph on land.

Even though the noun "hippopotamus" has a Greek root, it is often assigned the Latin plural "hippopotami." So, the plural form "hippopotami" is technically an error. Nevertheless, the word "hippopotami" has become an acceptable plural for "hippopotamus." (Of note, the word "hippopotami" is often used by pedants with a highbrow tone, believing it to be correct. It is therefore ironic that the term is based on an error. "Hippopotami" is also used by some as a joke plural.)

The plural "hippopotamuses" (which adheres to the standard rules for forming plurals) is more common than "hippopotami."

The shortened version "hippos" is often used in informal writing to avoid to making a decision on "hippopotamuses" and "hippopotami."

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The Standard Rules for Forming the Plurals

The table below shows the standard rules for forming the plurals of nouns in English.

Type	Example of Type	Forming the Plural	Plural
Most Nouns	lamp scythe	add s	lamps scythes
Noun Ending s, sh, ch, x or z	hippopotamus dress	add es	hippopotamuses dresses
Nouns ending [consonant] o	hero zero tomato	add either s or es (There are no rules for this - you have to know.)	heroes zeros tomatoes
Nouns ending [vowel] o	patio ratio	add s	patios ratios
Nouns ending [consonant] y	story penny	change the y to an i and add es	stories pennies
Nouns ending [vowel] y	donkey chimney	add s	donkeys chimneys
Nouns ending f or fe	dwarf hoof	ves and/or s (There are no rules - you have to know.)	dwarfs hooves or hoofs
Exceptions	man louse	some nouns undergo a vowel or letters change	men lice
More exceptions	salmon sheep	some nouns do not change at all	salmon sheep
Foreign rulings	hippopotamus medium	some nouns adopt foreign rulings	hippopotami media

Why Is There Confusion over the Plural of Hippopotamus?

Confusion arises over the plural of "hippopotamus" because the erroneously assigned Latin plural "hippopotami" grew in popularity, most probably because it sounds funny and clever. Nowadays, however, native English speakers are more drawn to "hippopotamuses," which adheres to the standard ruling for forming plurals.

"Hippopotamuses" is the more common plural of the two. (See the difference in Google's Ngram Viewer.)