Bava Metzia7: 8
שׁוֹמְרֵי פֵרוֹת אוֹכְלִין מֵהִלְכוֹת מְדִינָה, אֲבָל לֹא מִן הַתּוֹרָה.  אַרְבָּעָה שׁוֹמְרִין הֵן: שׁוֹמֵר חִנָּם, וְהַשּׁוֹאֵל, נוֹשֵׂא שָׂכָר, וְהַשּׂוֹכֵר. שׁוֹמֵר חִנָּם נִשְׁבָּע עַל הַכֹּל, וְהַשּׁוֹאֵל מְשַׁלֵּם אֶת הַכֹּל. וְנוֹשֵׂא שָׂכָר וְהַשּׂוֹכֵר נִשְׁבָּעִים עַל הַשְּׁבוּרָה, וְעַל הַשְּׁבוּיָה, וְעַל הַמֵּתָה, וּמְשַׁלְּמִין אֶת הָאֲבֵדָה וְאֶת הַגְּנֵבָה.
Shomrim of produce may eat by virtue of common custom, but not by virtue of Biblical law.  There are four [types of] shomrim: an unpaid shomer, a borrower, a paid shomer, and a renter. An unpaid shomer swears for everything; a borrower pays for everything. A paid shomer and a renter swear concerning an animal that broke a limb, an animal that was captured, and an animal that died, and they pay for loss and theft.
Bava Metzia7: 9
זְאֵב אֶחָד אֵינוֹ אֹנֶס; שְׁנֵי זְאֵבִים — אֹנֶס. רַבִּי יְהוּדָה אוֹמֵר: בִּשְׁעַת מִשְׁלַחַת זְאֵבִים — אַף זְאֵב אֶחָד אֹנֶס. שְׁנֵי כְלָבִים אֵינוֹ אֹנֶס. יַדּוּעַ הַבַּבְלִי אוֹמֵר מִשּׁוּם רַבִּי מֵאִיר: מֵרוּחַ אַחַת — אֵינוֹ אֹנֶס; מִשְּׁתֵּי רוּחוֹת — אֹנֶס. הַלִּסְטֵם — הֲרֵי זֶה אֹנֶס. הָאֲרִי, וְהַדֹּב, וְהַנָּמֵר, וְהַבַּרְדְּלָס, וְהַנָּחָשׁ — הֲרֵי זֶה אֹנֶס. אֵימָתַי? בִּזְמַן שֶׁבָּאוּ מֵאֲלֵיהֶן. אֲבָל אִם הוֹלִיכָן לִמְקוֹם גְּדוּדֵי חַיָּה וְלִסְטִים — אֵינוֹ אֹנֶס.
One wolf is not an unavoidable accident; two wolves is an unavoidable accident. R’ Yehudah says: At a time when wolves are sent out, even one wolf is an unavoidable accident. Two dogs is not an unavoidable accident. Yaddua the Babylonian says in the name of R’ Meir: From one direction, it is not an unavoidable accident; from two directions, it is an unavoidable accident. A brigand is an unavoidable accident. A lion, a bear, a leopard, a bardelas, or a snake is an unavoidable accident. When? When they came of themselves. But if he led them to a place [frequented by] bands of beasts or brigands, it is not an unavoidable accident.