- “I am a nazir if this animal is a hayya (a wild animal),” or
- “I am a nazir if this animal is a behema (a domesticated animal),” or
- “I am a nazir if this animal is both a hayya and a behema.”
In all of these cases the Mishna rules that the person becomes a nazir.
According to the conclusion of the discussion in Massekhet Bikkurim, the koy is recognized by the halakha for its unique status. Regarding some laws it is considered a hayya, with regard to others a behema, and regarding some others it appears to be a unique creation. Many of the commentaries understand our Mishna as ruling that the person in our case becomes a nazir min ha-safek – because of the doubtful status of the koy. The Rambam, however, believes that the person is not referring to the true definition of the koy, but rather to its status according to Jewish law. Since Jewish law treats the koy as a hayya with regard to some laws and as a behema with regard to others, an element of his statement is true, thus making him a nazir.
Koy – כוֹיּ – 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.