Gemara

(Mishnah): If one gave an animal to a Shomer Chinam, borrower, Shomer Sachar, or renter, the Shomer is like the owner. A Mu'ad pays full damage, and a Tam pays half-damage.

(Beraisa): Any of the four Shomrim is in place of the owner. If a Tam killed, it is killed, and Kofer is not paid. If it was Mu'ad, it is killed, and Kofer is paid. The Shomer pays the owner for the loss of his ox, except for a Shomer Chinam.

Question: If he guarded the ox, no Shomer should be liable. If he did not guard it, even a Shomer Chinam should be liable!

Answer: The case is, he guarded it minimally. This is enough for a Shomer Chinam, but not for other Shomrim.

(R. Elazar): If Reuven handed his ox over to a Shomer Chinam, and it damaged, the Shomer is liable. If it was damaged, he is exempt.

Question: If he accepted to guard it, he should be liable even if it is damaged! If he did not accept to guard it, he should be exempt even if it damaged!

Answer (Rava): He knew that it was a gorer. He accepted to guard it, i.e. from damaging, but did not think that he must guard it from being damaged!

55b: A minimal guarding suffices for Shen and Regel.

Rishonim

The Rif brings the Mishnah.

Nimukei Yosef: The Shomer is in place of the owner when he did not guard at all. We cannot say that he is liable like the owner, for all agree that a minimal guarding does not suffice for a Tam, but it suffices for a Shomer Chinam.

Rambam (Hilchos Nizkei Mamon 4:4): If one gave an animal to one of the four Shomerim, the Shomer is like the owner. If it damaged, the Shomer is liable. This is if he did not guard it at all. If he guarded it properly and it left and damaged, the Shomer is exempt and the owner is liable, even if it killed a person.

Ra'avad: This can only be if the owner knew (that the animal left), like we say about partners in a pit. However, this is not clear. A Shomer is responsible for a limited time, but partners are forever responsible. Therefore, if a Shomer guarded it properly and it left and damaged, he is liable only from when he found out, and the owner is exempt. However, the owner must always pay the victim, and then the owner claims from the Shomer. If he was a Shomer for a limited time, after the time he is exempt and the owner is liable.

Question (Tur): When the Shomer guarded properly, why is the owner liable? It seems that the Ra'avad challenged this.

Magid Mishneh: The Rambam obligates only Keren, i.e. things for which it is Tam. One is liable for these even if he guarded properly, like the Rambam says (7:1). He is exempt for Shen and Regel, like the Gemara says (55b). A minimal guarding is a door that can stand in a normal wind. A proper guarding is a door that can stand in any wind. This exempts Shomrim. Some say that a borrower is liable, for he is liable for Ones. One can distinguish.

Kesef Mishneh: In a Teshuvah, the Rambam explained that the correct text of the Mishnah Torah says that if the Shomer guarded properly, he is exempt. If he did a minimal guarding, a Shomer Chinam is exempt and the owner is liable, and any other Shomer is liable.

Beis Yosef (CM 396 DH u'Mah she'Chosav Shomruhu): Rashi, Tosfos and the Rashba obligate a borrower. The Rashba obligates a Shomer Sachar or renter to return the animal even if he did a proper guarding, even if he put an iron wall around it, unless he sat and guarded it. A proper guarding exempts all Shomrim from Kofer. This is why the Beraisa discusses a minimal guarding. Alternatively, it teaches that even a minimal guarding exempts a Shomer Chinam. The Rambam, Tur and Magid Mishneh obligate a borrower for damage to the animal b'Ones, but not for damage of the animal through Ones. L'Halachah, we are lenient not to obligate him, like the Rambam.

Rambam (ibid.): If he guarded it minimally, if he is a Shomer Chinam he is exempt. If he is a borrower, Shomer Sachar, or renter, he is liable.

Rosh (5:4): R. Elazar holds like Chachamim, who say that Stam, one who allows an animal into his Reshus accepts to guard it. This includes guarding it from being damaged (unless he knew that it is a gorer).

Poskim

Shulchan Aruch (CM 396:8): If one gave an animal to one of the four Shomrim, Stam the Shomer accepted to guard it from damaging and from being damaged. This refers to a normal ox. If he knew that it was a gorer, he accepted only to guard it from damaging, but not from being damaged.

Beis Yosef (DH Mosro, citing Rashi): It never crossed the Shomer's mind to guard a gorer from damage, for other animals fear it and do not go near it. R. Elazar discussed a Shomer Chinam, but the same applies to all Shomrim.

Shulchan Aruch (ibid.): If he did not guard it at all and it left and damaged, the Shomer is liable in place of the owner. He pays half-damage for a Tam, and full damage for a Mu'ad. The owner is exempt. After the Tam damaged, he cannot tell the owner 'the animal is in front of you', for Beis Din collects from its body.

SMA (16): The owner is exempt, for he set up someone to guard in place of himself. What more should he have done?!

Shulchan Aruch (ibid.): If he guarded it properly, he is exempt and the owner is liable only for Keren, which is Tam.

Question (SMA 18): The Beis Yosef said that the Magid Mishneh answered the Tur's question; one is liable for Keren even if he guarded properly. The Rambam (7:1) and Shulchan Aruch (396:1) exempt! In the correct text of the Rambam, 'the owner is liable' refers to a minimal guarding. The printer saw the mistaken text and 'corrected' the Magid Mishneh. The Magid Mishneh should say 'if he guarded minimally, a Shomer Chinam is exempt and the owner pays only Keren.' He proves this from Halachah 7:1; the Rambam obligates an owner who did a minimal guarding. Why did the Shulchan Aruch here rule like mistaken text of the Magid Mishneh, but in Sa'if 1 he himself exempts if he guarded properly (like the correct text of the Rambam)?!

Answer (Gra 18): The correct text of the Shulchan Aruch should say 'if he guarded it properly, he and the owner are exempt. If he guarded it minimally, if he is a Shomer Chinam, the owner is liable. If he is any other Shomer, the Shomer is liable.

Shulchan Aruch (ibid.): For Shen and Regel, also the owner is exempt. If he guarded it minimally, if he is a Shomer Chinam he is exempt. If he is a borrower, Shomer Sachar, or renter, he is liable.