פירוש על ביכורים 2:11

לא כתב לו את הכוי – Even though he wrote both of them, he (i.e., the father) only had his mind on a [what was] a definite cow and a definite chase/deer. And Maimonides explained that if he (i.e., the father) had written (to his son) only his cattle, he (the son) did not acquire the kind of bearded deer or antelope/כוי , for when we say to him (the father), bring proof that it is cattle; for if he had written [to his son] only his chase/deer, we say to him, bring proof that it is chase/deer. And it appears from his words that if he wrote both of them, he acquired them, from whichever side you take.

Introduction The final mishnah of this chapter discusses ways that a koy is either different from both a behemah and a hayyah and ways that it is similar to both.

אם אמר הריני נזיר שזה חיה או בהמה – Whether he said I am a Nazirite if this is a deer, or whether he said I am a Nazirite if this is cattle, he is a Nazirite, because in the question of doubt as to whether one has violated a negative commandment, it is judged stringently. For even if he stated, Behold, I am a Nazirite if this is deer and cattle, or this neither deer nor cattle, this is a doubtful Nazirite [and judged stringently].

And how is [a koy] neither like a behemah nor like a hayyah?
It is forbidden because of kilayim [to yoke it] with either a behemah or a hayyah. It is forbidden to yoke together two animals of different species. Since a koy might be a behemah, it cannot be yoked with any hayyah, and since it might be a hayyah, it cannot be yoked together with any other behemah.

One who deeds his son his behemah and his hayyah he has not [thereby] given him the koy. If one writes a will to his son, bequeathing to him his behemot and his hayyot, he has not bequeathed to him his koy, because a koy is neither a behemah or a hayyah.

If one says, “I will become a nazirite if this is [not] a hayyah or a behemah”, he is a nazirite. In all other ways it is like a behemah and a hayyah: The person here makes a bet that an animal approaching is either a behemah or a hayyah. If he loses his bet, he will be a nazirite. Although this would seem to be a hard bet to lose, somehow he does lose the bet, for a koy is indeed, neither a behemah nor a hayyah.

It requires slaughtering ( like them both; The mishnah now begins to list ways in which a koy has the same rules that apply to a behemah and to a hayyah. The first is that one who wishes to eat a koy must slaughter it in the appropriate manner.

It carries carrion impurity; Carrion (nevelah) is an animal that was not slaughtered in a proper manner. The nevelah of a kosher animal is impure, and since the koy is kosher, its nevelah is impure.

And to it applies the law relating to a limb of a living being like them both. It is forbidden to eat the limb of a living animal. This law applies only to beasts and wild animals but not to reptiles and other non-mammals. It applies to the koy in the same way that it applies to all behemot and hayyot.