We have learned that the Torah requires that a firstborn donkey be exchanged for a lamb that is given to the kohen(Shmot 13:13). The Mishnah on today’s daf (=page) teaches that either a lamb or a kid would be appropriate for such an exchange, but a calf or a wild animal could not be used. Similarly, a hybrid animal or a koy cannot be used.
A sheep–goat hybrid is the hybrid offspring of a sheep and a goat. Although sheep and goats seem similar and can be mated, they belong to different genera in the subfamily Caprinae of the family Bovidae. Sheep belong to the genus Ovis and have 54 chromosomes, while goats belong to the genus Capra and have 60 chromosomes, which often leads to stillborn offspring. Nevertheless, there are contemporary cases of living animals born from sheep and goats that mate.
According to the conclusion of the discussion in Masechet Bikkurim, the koy is recognized by the halakhah for its unique status. Regarding some laws it is considered a hayyah, with regard to others a behemah, and regarding some others it appears to be a unique creation.
Identifying the koy is a difficult task. Even though it is mentioned many times in the Mishnah and Talmudic literature, that is not because it is a common animal, rather because its status between a wild and domesticated animal allows it to serve as a test case for many halakhot. The disagreement as to its identification began in the time of the Mishnah, when some of the Sages argued that it is the offspring of a deer or similar animal with a goat. Others claim that it is a unique type of animal – an Ayal ha-bar.
The Ayal ha-bar can be identified with the ovis musimon, which, according to many, is the forerunner of domesticated cattle. It is distinguished by its short hair and grey color, and it lives in mountainous regions, where it is a nimble climber – today mainly in uninhabited areas in Europe. 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.
Its name, “koy” and even the pronunciation of the name, are themselves the subject of disagreement.