(Exodus 21:29) introduces the mued — the habitual goring ox: "And if it were a goring ox." The Mekhilta explains that this verse exists to draw clear distinctions between the tam (first-time gorer) and the mued. The two categories look similar on the surface — both involve an ox that kills — but their legal consequences differ dramatically.
The Mekhilta lists the differences. A mued requires that the owner be formally warned before witnesses. A tam does not require prior warning. A mued's owner pays kofer (ransom) when the ox kills a person. A tam's owner does not. A mued's owner pays thirty sela when the ox kills a bondservant. A tam's owner does not. A mued's owner pays full damages when the ox injures another animal. A tam's owner pays only half-damages. A mued's owner pays damages from his choicest property — the aliyah. A tam's owner pays only from the proceeds of selling the ox itself.
These distinctions create a complete legal framework. The tam is treated leniently because its violence was unexpected — no one could have predicted the first attack. The mued is treated severely because its owner was warned and failed to control a known danger. The Torah does not punish the animal's nature. It punishes the owner's negligence. Once you know your ox is dangerous and you fail to act, the full weight of the law shifts onto your shoulders.