Tractate Bava Metzia - Chapter 7 - Mishnah 9 בבא מציעא פרק ז׳ משנה ט׳

If one wolf [attacked the flock that he was watching] it does not count as an unavoidable accident [for which no blame is placed on the guardian]. Two wolves do count as an unavoidable accident. Rabbi Judah says: “In a time where wolves are commonly attacking the settlements, even one wolf is considered to be an unavoidable accident.” Two dogs do not count as an unavoidable accident. Yadua the Babylonian said in the name of Rabbi Meir says: “If [two dogs came] from one direction they do not count as an unavoidable accident, but if [they came] from two directions they count as an unavoidable accident. A bandit counts as an unavoidable accident. A lion or a bear or a leopard or a panther or a serpent counts as an unavoidable accident. When [is this so]? When they come of themselves. But if he took the flock to a place of wild animals or bandits they do not count as an unavoidable accident.