On Yoma 74:
איתיביה רבי יוחנן לריש לקיש אין לי אלא כל שישנו בעונש ישנו באזהרה כוי וחצי שיעור הואיל ואינו בעונש יכול אינו באזהרה תלמוד לומר כל חלב מדרבנן וקרא אסמכתא בעלמא
Rabbi Yoḥanan raised an objection to the opinion of Reish Lakish from what was taught in a baraita with regard to the prohibition of forbidden fat: I have derived only that anything that is included in the punishment of karet is included in the prohibition. However, one might have thought that there is no prohibition to eat fat of a koy, or a half-measure of forbidden fat, since there is no punishment for those. Therefore, the verse states: “ All fat” (Leviticus 7:23), indicating that there is a prohibition to eat any kind of fat, including fat of uncertain status and a half-measure of fat. Therefore, a half-measure of fat is prohibited by Torah law. Reish Lakish rejects this argument: This prohibition is rabbinic, and the verse brought as a proof is a mere support. It cannot be claimed that there is such a prohibition by Torah law.
Why would the brayta put together koy and chatzi shiur. Is the focus of Rabbi Yochanan's question just the latter part, the chatzi shiur, but these are just linked together in the derasha?
We could possibly answer based on Rav Chisda's understanding of a kvi / koy. On Chullin 79b, he defines it as a hybrid of a he-goat and a doe. Thus, it has aspects of both. And only the chelev of beheimot, not chayot, are prohibited. This is not a matter of safek, but rather of an instrinsic chatzi shiur.
