Would you say as stubborn as a mule, as one says in French, "têtu comme une mule" or cool as a mule ? I find mules to be sweet animals. We used to see them a lot in the Andalusian countryside and little by little, they seem to vanish from the landscape. We stumbled on that sweet one, last sunday in the village of Saleres.
oops...looking more carefully I think this one is a donkey, isn't it ?