According to the Torah (Shemot 13:13) firstborn male children and kosher animals are consecrated to the Temple from birth, and must be redeemed. There is one non-kosher animal that also must be redeemed by exchanging it for a seh – a lamb. Our Gemara quotes a Mishna in Massekhet Bekhorot (1:5), which teaches that the exchange cannot take place with just any animal – it cannot be exchanged for a calf or a non-domesticated animal; it cannot be exchanged for a seh that had already been slaughtered, an animal that is clearly a terefah (it has a physical condition that will cause it to die within a year) or with a koy.
כוֹיּ – koy – refers to an animal that has the features of both a wild animal and a domesticated one. Many problems arise in trying to identify the koy. It is mentioned numerous times in the Mishna and the Gemara, not because it is a common animal, but rather because it is useful in discussion that explore the parameters and limits of the laws of domestic animals versus wild animals, and allows it to be a test case for many halakhot. As early as the mishnaic period, the Sages disagreed on the identification of the koy . Some maintain that it is a hybrid born to a deer, or another kosher wild animal, and a goat.
According to many researchers, the koy is identified as the water buffalo. There are allusions to this identification in some medieval rabbinic sources. Others reject this idea and claim that water buffalo did not live in Eretz Yisrael during the time of the Mishna, when the koy was first mentioned. Others maintain that the koy is a unique type of animal – an Ayal HaBar.
The Ayal HaBar can be identified with the mouflon sheep, Ovis musimon, which, according to many, is the forerunner of domesticated sheep. The mouflon is a mountain-dwelling long-haired wild sheep subspecies, distinguished by its short hair and grey color; a nimble climber, it lives in mountainous regions, today mainly in uninhabited areas in Europe. While there are a number of opinions as to the specific subspecies of mouflon a koy may be, it is likely that the clear similarities between a koy and a sheep, together with its being a wild animal, led to the Sages’ confusion about its classification. There is also uncertainty with regard to both the origin of the term koy and its proper vocalization, themselves the subject of talmudic disagreement.