You may have heard that English is a Germanic language. The -en ending on plurals is something we get from our German roots. In Old English, some nouns were made plural with -s and -es as they are today, but many nouns took -en to become plural. ... Today only a few en-forms survive; the most common are oxen and children.

Is it ox or oxes?

According to a English grammar rules, we add 'es' when the noun ends with 's', 'ss', 'sh', 'x', or 'ch'. With these rules, 'ox' would become 'oxes' in the plural.

What is the plural form of ox '?

noun. \ ˈäks \ plural oxen\ ˈäk-​sən \ also ox.