Bava Metzia, Chapter 7, Mishnah 9. This mishnah brings concrete examples of what is considered an ones (an unavoidable accident) and what is not considered an ones, all dealing with a shepherd guarding a flock. The shepherd is a shomer sachar - he is paid for his guarding services - and therefore he bears increased responsibility.
One wolf and two wolves:
"Zeev echad eino ones" - one wolf is not an unavoidable accident - a lone wolf approaching the flock is not considered an ones, since the shepherd is expected to try and fend it off with a stick or something similar. The Mishnah's assessment is that this is a manageable situation: if he did not try to chase it away and the wolf preyed upon a sheep - the shepherd is liable, and he cannot claim that it was an ones. However, if he tried to chase it away and was unsuccessful - it is an ones.
"Shnei zeevim ones" - two wolves are an unavoidable accident - the Mishnah concedes that one cannot be expected to stand against two wolves. It is almost certain that he will not prevail, and therefore he has no obligation to try; he may even flee, and this is not held against him. It is an ones, and he is exempt.
It is worth noting that the wolves found in the Land of Israel are similar to large dogs, and are not the giant wolves of North America or Russia. They are smaller than them.
Rabbi Yehudah disagrees and holds: "Bishaas mishlachas zeevim af zeev echad ones" - during an outbreak of wolves, even one wolf is an unavoidable accident. In a scenario where the wolves go out and prowl for prey, even a lone wolf is considered a circumstance beyond the shepherd's control, and he is not obligated to try to fend it off - and he is exempt.
The word "mishlachas" is a specific term, and it means that the wolves are sent out. We find a similar usage regarding the plague of arov, as it states: "Behold, I will send [mashliach] the arov against you." It is the same idea: when the animals go out to seize the flock, there is nothing a person can do.
However, the halachah is not like Rabbi Yehudah, but rather like the Tanna Kamma: one wolf can be chased away.
Two dogs:
"Shnei kelavim eino ones" - two dogs are not an unavoidable accident - even against two stray dogs, wild dogs approaching the flock, the lone shepherd is expected to stand his ground and make an effort. If he tried and was unsuccessful - it is an ones; if he did not try at all - the blame is on him. It goes without saying that three dogs are beyond his strength, and he is not expected to even try.
Yadua HaBavli - this is his name, and his title is based on his origin from Bavel - says in the name of Rabbi Meir, meaning in the name of Rabbi Meir based on a tradition he had, and not that he heard it from him directly: "Meruach achas eino ones" - from one direction, it is not an unavoidable accident. If the two dogs come from one direction, he can do something, and he cannot claim that the matter was beyond his control. But "mishtei ruchos" - from two directions - when they approach the flock from two separate directions, what can the shepherd do? It is an ones. Here too, the halachah is not like Yadua, but rather he is expected to chase away two dogs or at least make an effort, regardless of where they came from.
Armed robbers:
When armed robbers arrive - and even a single armed robber appearing with his weapon in hand - it is an ones, a circumstance beyond the shepherd's control. The shepherd is not expected to risk his life for the flock against someone who clearly intends to fight for his life with the weapon in his hand.
It is worth making a distinction: We said that a paid watchman is expected, to a certain extent, to even risk himself to deal with a wolf - but not against an armed robber. An unpaid watchman, on the other hand, is not expected to risk his life in any way. Therefore, an unpaid watchman may flee even from a single wolf and be exempt, not necessarily because it is an unavoidable accident, but because it falls under the category of theft and loss, for which he is also not responsible.
The Five Dangerous Animals:
When the Mishnah mentions dangerous animals, it usually lists a group of six. The wolf is one of them, and here the other five are listed: "Ha'ari vehadov vehanamer vehabardelas vehanachash - harei zeh oness" - the lion, the bear, the leopard, the cheetah, and the snake are an unavoidable accident. These five animals are dangerous, and therefore if they appear, the shepherd is permitted to flee and is exempt, and he is not expected to fight them.
"Ha'ari" - the lion. Lions were found in the Land of Israel in the past, apparently until about a century ago.
"Hadov" - the Syrian bears that were in the north of the country. They too are no longer found in Israel.
"Hanamer" - the leopard, those large spotted cats. There are still a few dozen Arabian leopards in the south of the country, in the Negev. Some mistakenly think this refers to a tiger, but there are no tigers in Israel and there never were.
"Habardelas" - a word that is difficult to translate. In the Babylonian Talmud, as well as in the Rambam and others, it is explained as a hyena. Two types of hyenas are found in the Land of Israel: the spotted ones, large and frightening like those in South Africa, which are very dangerous; and the striped ones, which are smaller and less dangerous but dangerous enough, and can still be found here today (and they have even been seen locally). In any case, "bardelas" is not a Hebrew word but is borrowed from Greek: the word "pardelas" referred to a leopard. Since we cannot say the reference here is to a leopard, as the leopard is already listed on its own, it likely refers to a cheetah - a smaller spotted cat that was found in the Land of Israel in the past but is no longer here.
"Hanachash" - the snake, as it sounds.
Leading Them to a Place of Danger:
When is the shepherd permitted to claim that it is an unavoidable accident, when he encounters armed robbers or these vicious animals? When they came upon him on their own, for example, if they attacked at night. But if he himself led the animals to a dangerous place - he knows where the lions' den is, he knows that robbers roam in a certain cave, and yet he passed by it - he cannot claim that the matter was out of his control. On the contrary, it was in his control not to go to the place of danger, and therefore he will be liable.
In summary: In this Mishnah we learned the limits of an unavoidable accident for a shepherd who is a paid watchman: one wolf as well as two dogs are not an unavoidable accident, and he is expected to try to fend them off; two wolves, three dogs, armed robbers, and the five dangerous animals - the lion, the bear, the leopard, the cheetah, and the snake - these are unavoidable accidents and he is exempt. We discussed the opinion of Rabbi Yehudah regarding "a time of wolf packs" and the opinion of Yadu'a the Babylonian in the name of Rabbi Meir regarding dogs coming from two directions, that the halachah does not follow either of them; the distinction between a paid watchman and an unpaid watchman; and the concluding rule - that the entire exemption for an unavoidable accident applies only when they come upon him on their own, but one who leads his flock to a place of danger is liable.