Answer

The Talmud in Bava Kamma 15a brings a dispute about this, which depends on the question whether an ordinary ox is presumed to be guarded or not. If it is not presumed to be guarded, then the half-damages are an ordinary monetary liability for damages, except that the Torah had pity on him and reduces half of the payment, since after all this is an ox that is not established as prone to damage. Or, strictly speaking, he would have been exempt because an ox is presumed to be guarded, and the Torah fines him in order to motivate him nevertheless to guard it. According to this view, it is a fine. In practice, Jewish law rules in accordance with the second opinion.