The Mishna in the beginning of Bava Kama itself is clear that the obligation of guarding an animal so that it will not go out and cause damage is on the person in whose domain the animal is, not the owner. This point is brought in Rav Shach in the beginning of Laws of Damages. But this is contradicted by a braita in Bava Kama page 45 that says there are four types of guards, free, for pay, a renter, and a borrower. If the animal goes and kills someone, they all have to bring the kofer [restitution] and if the animal has already acquired a status of being liable to kill, it is killed, and the guards pay the cost of the animal to the owner except for the unpaid guard. The gemara there says the case is where each one guarded the animal with minimum confinement. If the there would be an obligation on the unpaid guard to pay damages, then he would not be exempt from the cost of the animal. Rather we see it is the owner who pays damages. The other guards would pay back the owner, but not the unpaid guard. The reasoning here is if the case was not of killing but of the animal causing damage, and the unpaid guard had an obligation to people that the animal caused damage to, then he would be required to do maximum confinement like the other guards. This point is brough by Rav Shach in his letter to Rav Isar Meltzer in relation to understanding the opinion of the Ramban in his commentary on Bava Metzia page 36 and page 93. however i intend here to show that this does not depend on the Ramban, but rather is a direct contradiction between the mishna and the braita on page 45/obligation of damages/. I might mention that the Rambam in laws of damages chapter one goes like the braita that the obligation for damages is upon the owner of the animal. --------------------------The משנה in the beginning of בבא קמא itself is clear that the obligation of guarding an animal so that it will not go out and cause damage is on the person in whose domain the animal is, not the owner. This point is brought in רב שך in the beginning of הלכות ניזקי ממון. But this is contradicted by a ברייתא in בבא קמא page מ''ה that says there are four types of guards, free, for pay, a renter, and a שואל. If the animal goes and kills someone, they all have to bring the כופר [restitution] and if the animal has already acquired a חזקה of being liable to kill, it is killed, and the guards pay the cost of the animal to the owner except for the שומר חינם. The גמרא there says the case is where each one guarded the animal with minimum confinement(שמירה פחותה) . If the there would be an obligation on the שומר חינם to pay damages, then he would not be exempt from the cost of the animal. Rather we see it is the owner who pays damages. The other guards would pay back the owner, but not the שומר חינם. The reasoning here is if the case was not of killing, but of the animal causing damage, and the שומר חינם had an obligation to people that the animal caused damage to, then he would be required to do maximum confinement שמירה מעולהlike the other guards. This point is brough by רב שך in his letter to ר' איסר מלצר in relation to understanding the opinion of the רמב''ן in his commentary on בבא מציעא page ל''ו and page צ''ג. however I intend here to show that this does not depend on the רמב''ן, but rather is a direct contradiction between the משנה ארבעה אבות נזיקים and the ברייתא on page מ''ה------I might mention that the רמב''ם in הלכות נזיקים פרק א' goes like the ברייתא that the obligation for damages is upon the owner of the animal