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tête de mule (addressing the stubborn mule), têtu/têtue comme une mule (describing someone as stubborn as a mule) are standard French expressions.
The Mule has a reputation for being stubborn, hence the saying "Stubborn as a mule"
it means to be very stubborn. exp-my bro was as stubborn as a mule and refused to buy me stuff
mule
The sentence 'Stubborn as a mule' is a simile as it contains the word 'as' which makes it a comparison.