We'll discuss a case in which a person gave his animal to somebody else to watch, and that person assumes responsibility for it. The Mishnah says that if an owner of an animal handed over possession of his animal to the four different types of Shomrim (watchmen) that the Torah mentions: a Shomer Chinam, who is an unpaid watchman. He agrees to watch the animal without being paid. A Shomer Sachar: you're paying that person to watch your animal for you. A Socher, a renter, is renting that animal and is allowed to use it. He's renting the animal from the owner. Or a Shoel: he's borrowing the animal. He's allowed to use that animal for free. All of these people take the place of the owner. They're responsible for the damage as if they were the owner. If the animal was Mu'ad (forewarned), then any one of the Shomrim here would pay the full damages. But if the animal was a Tam (unwarned), then they would only pay Chatzi Nezek (half damages), like the regular owner. If the owner did certain things to prevent the animal from doing any damage, we'll talk about how they would not be obligated to pay if the animal did cause certain types of damage. We'll talk about a three-way machloket (dispute), and we'll look at what type of preventative measures have to be taken to poter the owner, to exempt the owner from having to pay if damage was done.
First opinion: If the owner tied it up with a leash, which provides a minimal amount of protection against its breaking free and causing damage, or locked the gate properly in front of it. There was a small gate, a regular pen that would stand up to a normal wind. Let's say the animal got out—either it tore the leash, which wasn't a super-strong leash, or a strong wind came and knocked the gate down—and did damage. Whether the animal was a Tam animal or a Mu'ad animal, the owner would be obligated to pay Chatzi Nezek for a Tam and Nezek Shalem (full damages) for a Mu'ad. This is the opinion of Rabbi Meir. According to him, the animal has to be guarded in a super-strong way in order to exempt the owner from having to pay these damages. It has to be either secured with an iron leash around it, so that it's impossible for it to get out, or locked down with a chain so that no matter how hard the animal pulls, it wouldn't be able to get out; or it has to have a very strong door that even a super-strong wind couldn't knock down. That's the only circumstance, according to Rabbi Meir, in which the owner would not be obligated to pay damages if the animal got out. But if the animal was guarded in a normal way and got out and did damage, according to Rabbi Meir, the owner would still be liable.
According to Rabbi Yehuda, if the animal is a Tam animal, then of course the owner has to pay the half damages, like in Rabbi Meir's opinion. Where Rabbi Yehuda does argue is with a Mu'ad. If the animal was Mu'ad, the owner is not obligated to pay the full damages; he would still be obligated to pay the half damages. Why? Because the Pasuk (verse), when talking about a Mu'ad animal, says that the owner of a Mu'ad has to pay the full damages only, as the Pasuk says: if the owner didn't guard it. This tells us that he has to pay full damages only if it was not guarded at all. Ah. But in our case, the animal was guarded. It was guarded in a minimal manner, but that's still enough to tell us that he wouldn't be obligated to pay the Nezek Shalem, the full damages, if it was guarded in its usual, normal manner. That would be enough to say that the owner does not have to pay the 100% damages; he would only have to pay for the 50% damages.
A third opinion: Rabbi Eliezer omer. Rabbi Eliezer is very strict. He says that the only way to guard a Mu'ad animal is with a knife. In other words, to kill it. The only way to prevent a Mu'ad animal from causing damage is to slaughter it, to kill it. That's the only way you could prevent it from causing any more damage. So, if the owner had the animal, it doesn't matter whether he put a leash around it, an iron gate around it, or an iron chain; no matter what, you can't prevent this animal from causing damage again. Therefore, because he didn't slaughter it, he's obligated if it causes damage, and he has to pay in full, no matter how well he guarded this animal. So we have a three-way machloket about the Mu'ad shor (ox) over here. And we have to take note that we're only talking about Keren damage. We're only talking about on-purpose damage that this shor does. And when talking about Shen (tooth) or Regel (foot), everybody agrees that an ordinary level of guarding would be enough.