All of our words are irregular plural nouns. That means they do not follow any of the plural noun spelling rules.
men (plural fo man)
children (plural for child)
women (plural for woman)
geese (plural for goose)
mice (plural for mouse)
fish (same plural or singular)
teeth (plural for tooth)
feet (plural for foot)
deer (same plural or singular)
sheep (same plural or singular)