Hello thank you for confirming that information.
As far as the law is concerned, and it may sound silly, but pets, are considered as goods, under the consumer rights act 2015.
The issue then is whether this act applies to your sellier. If it does, then they would have sold a “defective puppy”.
What would need to be considered is whether they are classed as a private seller or business seller under the act.
It is not always clear, whether a “breeder” is a private or business seller.
So other things need to be considered.
The most important rule taken into consideration is, was the puppy sold for profit. If so that would suffice to be classified as a business seller. If the seller is also registered and likewise the puppies, that would help establish the matter.
Therefore, with all the above taken into consideration, it would most likely be accepted that they would be classed as a business seller, and that the consumer rights act 2015 would apply to them.
If therefore the puppy was sold as being in good health, and with the medical records stating the medical condition of the puppy was from birth, it could be held that the sale of a healthy puppy was a misrepresentation, and you could seek a refund and / or compensation.