In French, most nouns are pluralised by adding “s”  at the end of the noun. This is true for both masculine and feminine nouns:

the house – the houses > la maison – les maisons

the brother – the brothers > le frère – les frères 

However, there are a few exceptions to this simple “s” rule: if a noun ends in -eu, -au, or -eau, an “x” is added to express the plural: the castle – the castles > le château – les châteaux

If a noun ends in -al, this ending is replaced with “aux” to express the plural: the horse – the horses > le cheval – les chevaux 

If a noun ends in -s, -x or -z , there is no change in spelling when expressing the plural. The singular and plural forms are identical: the mouse – the mice > la souris – les souris

Some nouns are exceptions and require straightforward memorisation:

the eye – the eyes > l’oeil – les yeux

Mister – Misters > Monsieur – Messieurs