What is an irregular noun?

Irregular nouns have a plural that does not follow the standard rule. A regular noun is a plural is formed by adding an -s or -es to the singular, for example:

book/books

girl/girls

apple/apples

chair/chairs

dress/dresses

class/classes

wish/wishes

box/boxes

But some nouns are irregular, they form their plural in different ways, for example:

child/children

man/men

ox/oxen.

person/people

mouse/mice

goose/geese

foot/feet

tooth/teeth

cactus/cacti

knife/knives

medium/media

larva/larvae