Short answer. You must return the animal in the same condition it was in when it was sold and delivered to you. Under Article 1579 of the Civil Code, you are responsible for any injury caused by your own negligence — but not for deterioration that resulted from the hidden defect itself that led to the rescission.
What the law says
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 →
The baseline: return the animal as you received it
Article 1579 establishes a straightforward baseline for rescission of an animal sale: the animal goes back to the seller in the condition it was in at the time of the original sale and delivery. You are not required to improve its condition before returning it, but you are also not permitted to return it in a worse state than it was in when you took possession — unless that worse state traces directly to the redhibitory defect that got the sale rescinded in the first place.
You bear responsibility only for your own negligence
The article draws a clear line between two types of deterioration. Deterioration you caused through your own negligence — for instance, failing to provide proper feed, shelter, or veterinary care — is your financial responsibility. If the animal's condition worsened because of how you handled it, the seller can hold you answerable for that damage. But deterioration that arose from the very redhibitory fault or defect that led the court to rescind the sale is not your problem. You cannot be blamed for an injury the animal's own pre-existing condition caused.
Why this allocation makes sense
The logic of Article 1579 follows from the nature of rescission. When a court rescinds an animal sale because of a hidden defect, it is essentially saying the defect was so serious it should undo the transaction entirely. Holding the buyer liable for harm that flows from that same defect would be inconsistent — you would be penalized for an injury the seller's defective animal inflicted on itself. At the same time, the seller is entitled to get back an animal that has not been wasted through the buyer's careless treatment. Both sides are restored, as far as possible, to their pre-sale positions.
Practical steps when you are ordered to return an animal
If a court orders rescission, document the animal's current condition carefully — photographs, a veterinary examination, and a written report — before you return it. This protects you if the seller later claims you returned the animal in worse shape than it was received. If the animal's condition changed during your possession, be prepared to show that any worsening traces to the defect the court already recognized, not to your own handling. Keep records of the care you provided during your time with the animal. These records become your evidence if a dispute arises about the state of the animal on return.