Irregular plural nouns in English do not follow standard rules (adding -s or -es) and often require memorization.
Man - Men
Woman - Women
Mouse - Mice
Leaf - Leaves
Thief - Thieves
Knife - Knives
Focus - Foci
Child - Children
Analysis - Analyses
Phenomenon - Phenomena
Sheep - Sheep
Deer - Deer
Person - People
Die - Dice