מדרש אויף שמות 21:36

"and the owner were warned": We are hereby apprised that he is not liable unless he was warned. "and he did not guard it": to bring (even) a non-paid watcher. Variantly: "and he did not watch it": as befits it — whence they ruled: If he watched it as befits it, he is not liable; if not as befits it, he is liable. If he tied it with a harness and it freed itself and damaged, he is liable for both a tam and a mued, it being written (Ibid. 36) "and its owner did not guard it" — and this is not considered guarding. These are the words of R. Meir.

R. Yehudah says: For a tam he is liable; for a mued, he is not liable, it being written (of the latter) "and its owner not guard it" — and this is considered guarding.

"Money shall he restore to its owner': This tells me only of money. Whence do I derive the same for a beast? (i.e., that he may pay with a beast.) It follows, viz.: It is written here "and the carcass shall belong to him," and elsewhere, (Exodus 21:36) "Then shall he pay an ox for an ox, and the carcass shall belong to him. Just as there (he pays) a beast, so, here (he may pay) a beast. Just as here, money; so, there, money.