Most English nouns have the plural suffix -s or -es. Some plurals such as children, men and women are irregular. However, there are a few which are identical in both singular and plural. Here is a list of such nouns:
aircraft antelope bass bison carp
cod deer elk fish grapefruit
moose offspring salmon sheep series
shrimp trout tuna won yen
For the noun fish the plural fishes is also possible, but this form isn't so common and can be considered archaic. In the case of shrimp, many British English speakers use the form shrimps.
Many of these invariable nouns are types of fish, i.e., carp, salmon, tuna and names of animals, i.e., deer, moose, sheep. They can also be the names of currencies such as won and yen. The word fruit has the plural fruits, but in the compound grapefruit the noun is invariable in number.
A few English nouns are invariable in singular and plural. Many of these nouns are names of animals, fish and currencies. These nouns with identical singular and plural forms are known as invariable nouns.