Chullin, Chapter 6, Mishnah 1. This chapter turns to the mitzvah of kisui hadam, the covering of the blood. After a person performs shechitah on a kosher chayah, a wild undomesticated animal, or on an of, a bird, the Torah commands that the blood be covered with earth or with a similar substance. Our mishnah lays out the scope of this mitzvah: where it applies, when it applies, and to which creatures.
Where and when the mitzvah applies
"Kisui hadam noheg ba'aretz uv'chutz la'aretz." The obligation to cover the blood is in force in Eretz Yisrael and equally in the lands outside of it. It is not one of the mitzvot bound to the soil of the Land.
"Bifnei habayit veshelo bifnei habayit." It applied while the Beit Hamikdash stood, and it continues to apply now that the Beit Hamikdash is no longer standing.
"Bechullin aval lo bemukdashin." The mitzvah belongs to unconsecrated animals. An animal that had been dedicated to the Beit Hamikdash is excluded, and there is no requirement to cover its blood.
Which creatures are included
"Venoheg bechayah uv'of." The mitzvah governs wild animals and birds. These are precisely the two categories named in the pasuk, "chayah o of asher ye'achel," a wild animal or a bird that may be eaten, and the mitzvah is limited to them.
"Bimzuman veshe'eino bimzuman." It makes no difference whether the creature was already at hand, waiting in one's courtyard and ready to be slaughtered, or whether it had to be hunted and trapped first and only then brought to shechitah. Either way, the blood must be covered.
The koy and its doubtful status
"Venoheg bekoy, mipnei shehu safek." The mitzvah also applies to a koy, and the mishnah tells us why: the koy is a case of genuine uncertainty. We cannot determine whether it belongs to the category of chayah or to the category of behemah, a domesticated animal. The odds, so to speak, stand evenly balanced.
We do not actually know today what sort of creature the koy was, and various theories have been offered to identify it. What is clear is the halachic consequence: since this is a doubt in a Torah law, and safek de'oraita lechumra, we rule stringently, and its blood must be covered.
Slaughtering a koy on yom tov
"Ve'ein shochatin oto beyom tov." One does not slaughter a koy on yom tov. "Ve'im shechato, ein mechasin et damo." And if a person went ahead and slaughtered it anyway, the blood is not to be covered. Covering is permitted on yom tov only for a chayah or an of, whose status is certain.
The logic deserves attention. Covering the blood involves real exertion, yet Chachamim permitted that exertion on yom tov for the sake of the mitzvah. By that reasoning, covering the blood of a koy should have been permitted as well, since the koy may well be a chayah and the person may be fulfilling an actual mitzvah.
Chachamim nevertheless forbade it, out of concern for what onlookers would conclude. If they see the blood of a koy being covered, they will reason that the koy has been established as a chayah. Once that assumption takes hold, they will go on to eat its chelev, its forbidden fats, because the chelev of a chayah is permitted for consumption. But the chelev of a koy may not be eaten, precisely because the animal may turn out to be a behemah.
So the prohibition against covering the blood of a koy on yom tov serves as a standing reminder: the status of this creature remains unresolved, and its chelev stays forbidden to us.