Short answer. Only for injuries caused by your own negligence. Article 1579 requires the animal to be returned in the condition it was sold and delivered in, but you are answerable only for injury due to your negligence, not for anything arising from the redhibitory fault or defect that justified the rescission.
What the law says
If the sale be rescinded, the animal shall be returned in the condition in which it was sold and delivered, the vendee being answerable for any injury due to his negligence, and not arising from the redhibitory fault or defect.
Civil Code, Article 1579 — Return of the Animal on Rescission. Read the full provision →
You must return the animal, but you are not automatically liable for everything
Article 1579 sets out both your duty and its limit at the same time. It provides that if the sale be rescinded, the animal shall be returned in the condition in which it was sold and delivered, the vendee being answerable for any injury due to his negligence, and not arising from the redhibitory fault or defect. You are expected to return the animal in essentially the same condition it was in when you received it, but the article immediately narrows your liability for any injury that occurred while it was in your care.
Negligence on your part is what triggers liability
The standard the article sets is negligence, not strict responsibility for anything that happened to the animal. If the animal suffered an injury because of something you did or failed to do that a reasonably careful owner would have avoided, such as neglecting proper care or exposing it to an obvious hazard, you are answerable for that injury when you return the animal. Injuries that occurred despite reasonable care on your part are not something you have to answer for under this provision.
Injuries tied to the original defect are not your responsibility
The article draws a specific line excluding injury arising from the redhibitory fault or defect itself. Because the whole reason for the rescission is that the animal had a hidden defect the seller should have disclosed, any deterioration, injury, or complication that resulted from that same underlying defect is not something you caused and not something you have to answer for. The seller cannot use the very defect that justified rescission as a reason to reduce what they owe you or demand compensation from you.
What this means as you prepare to return the animal
When returning a rescinded animal, be ready to distinguish between changes in its condition that trace back to the original defect, for which you owe nothing, and any injury that resulted from your own carelessness while the animal was in your possession, for which you can be held liable. Keeping records of how you cared for the animal can help show that any deterioration was tied to the defect rather than to anything you did.