In the context of discussing animals from which one is obligated to offer the zero’ah (shoulder), lehayayim (cheeks) and kevah (stomach) to the kohanim the Gemara quotes a baraita that teaches that mixed breed animals as well as a koy – an animal that has the features of both a wild animal and a domesticated one– are obligated in these gifts, as well. Given the juxtaposition of koy with mixed-breed animals, it would appear that the koy is the result of a union between a two animals – a deer and a goat – but we have already learned that according to some of the Sages the koy is a beriah bifnei atzmah – it is a unique creation – about which the Sages could not conclude if it is a wild animal or a domestic animal (see above, daf or page 80).
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.