What is the metaphor of is an ox?

The question is illogical.

A metaphor is a figure of speech that says that one thing is another different thing. This allows us to use fewer Words and forces the reader or listener to find the similarities, e.g. Time is money, the whole world is a stage.

Is an ox is not a metaphor, nor does it contain a metaphor. It is simply part of a sentence containing a verb, an indefinite article and a common noun.