A agrees to buy from B a certain horse, it turns out that the horse was dead at the time of bargaining though neither party was aware of the fact.

As per Indian Contract Act, 1872-   Section 20.   Agreement void where both parties are under mistake as to matter of fact. Where both the parties to an agreement are under a mistake as to a matter of fact essential to the agreement, the agreement is void. Explanation- An erroneous opinion as to the value of the thing which forms the subject-matter of the agreement, is not to be deemed a mistake as to a matter of fact. Illustrations   (b) A agrees to buy from B a certain horse. It turns out that the horse was dead at the time of the bargain, though neither party was aware of the fact. The agreement is void.