Plural of Hippopotamus

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The Quick Answer
The plural of hippopotamus is hippopotamuses or hippopotami.

The Plural of Hippopotamus

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.)
Another Interactive Exercise
Here are three randomly selected questions from another exercise on plurals, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

See Also

Unusual pluralsPlural forming table Quirks with forming plurals Forming the plurals of abbreviations Forming the plurals of compound nouns