Irregular Noun Other nouns form the plural irregularly. For example: man/men, child/children, foot/feet, mouse/mice, ox/oxen, sheep/sheep, person/people. Since there is no âsâ at the ending of such plural nouns, you will have to add apostrophe âsâ (âs) to make it possessive.rregular Noun Other nouns form the plural irregularly. For example: man/men, child/children, foot/feet, mouse/mice, ox/oxen, sheep/sheep, person/people. Since there is no âsâ at the ending of such plural nouns, you will have to add apostrophe âsâ (âs) to make it possessive.