Up to this point we have differentiated between a shor tam and a shor mu’ad by describing the former as an ox without a violent history and the latter as an ox that has gored on at least three occasions. As the Mishna on today’s daf makes clear, the reality is quite a bit more complicated. According to the Mishna, an ox can be considered mu’ad with regard to other oxen, even though it is not mu’ad for other animals. Similarly, it may be mu’ad for people, but not for animals. The Mishna even relates that Rabbi Yehuda ruled that an ox that shows violent tendencies only on Shabbat will be considered mu’ad for Shabbat, but not for the rest of the week.
Why would the halakha make these distinctions?
Generally speaking, non-predatory animals are not attackers by their nature, and will not purposefully injure animals of a different type. When they do attack and injure, it is usually when they are in heat during mating season or to establish an area that they control. Thus, domesticated animals are ordinarily viewed as mu’ad only to animal of the same species, and only rarely will they be mu’ad towards other types of animals.
Regarding animals that are considered mu’ad only on Shabbat, the Talmud Yerushalmi explains that people dress differently on Shabbat, and an ox may not recognize the people around him, which may lead it to violence. (Tosafot Rabbeinu Peretz points out that according to this approach, this rule would apply only to an ox who is mu’ad to gore people, but not to one whose violence is directed at animals.) Rashi explains that since the ox is not working on Shabbat, it is the only day on which the animal is not tethered to something, and the freedom that it has may lead it to out-of-the ordinary acts of violence.