The Mishna (73b) taught that in order to be held liable for eating or drinking on Yom Kippur, one must consume a certain amount of food or drink. What if one eats less than that amount – is it still forbidden by the Torah, or is it permitted by the Torah (i.e. such eating is not significant as far as halakha is concerned) and only forbidden on a Rabbinic level? Rabbi Yohanan believes that eating less than the full amount is still forbidden by the Torah (although there will be no punishment for having done so); Reish Lakish believes that it is permitted by the Torah.
Rabbi Yohanan presents a baraita that challenges Reish Lakish’s position. The baraita teaches that helev – forbidden fats – cannot be eaten, even in a case where there is no punishment, like a case of a koy or eating less than a full amount, which is based on the passage that says kol helev (Vayikra 7:23) – any helev. Reish Lakish responds by saying that the law is Rabbinic in origin and the passage quoted is merely an asmakhta – a support used by the Rabbis for their rule.
The koy discussed here is an animal that is not clearly defined as either a behema (a domesticated animal) or a haya (a wild animal). The Gemara in Hullin discusses the difference between wild and domesticated animals in some detail. While it is clear that cattle (cows, sheep, goats) are domesticated animals, some of their close relatives are considered wild animals, and it is often difficult to draw a clear line of demarcation between, for example, a wild and a domesticated goat. On occasion, wild animals that are closely related to cattle are herded and raised together with domesticated ones. Those animals would fall into the halakhic quandary of the koy.
Given the unclear status of such animals, there are a number of laws that may or may not apply to them, like our case of helev (which is only forbidden in behemot – see 7:22-25), the case of kisuy ha-dam (covering the blood of an animal that is slaughtered, which applies only to a haya – see 17:13), etc.