כוי

לרפואת עמיחי ישראל בן לאה

Says the Rambam: 

הכוי חלבו מטמא כבשרו וטומאתו בספק לפיכך אין שורפין עליו תרומה וקדשים ואין חייבין כרת על טומאתו ועל ביאת המקדש או על אכילת קדשיו:

The fat of a ko'i imparts impurity like its meat does, but this impurity is of doubtful status. Therefore terumah and sacrificial foods are not burnt because of it, nor is one liable for kares for entering the Temple or partaking of sacrificial foods after touching it. [Shear Avos Hatumah 1-6] 

A כוי is a ספק חיה ספק בהמה. If it has the status of a חיה then it's חלב is טמא [as the Rambam taught in the previous halacha]. If it has the status of a בהמה, then it is not. 

The Achronim prove that the Rambam is of the opinion that a כוי is an independent species [sort of like my 5 year old daughter] and the rabbis didn't decide if it is a חיה or בהמה. 

If so, why did the Rambam not pasken in Hilchos Ma'achlos Asuros that if someone eats the חלב of a כוי he is עובר on the איסור of כל חלב [Vayikra 7-23] as it says in Yoma [74]:

אין לי אלא כל שישנו בעונש ישנו באזהרה, חלב כוי וחצי שיעור הואיל ואינו בעונש יכול אינו גם באזהרה ת"ל כל חלב

 

Indeed Tosfos [Yoma 74b, Krisus 21a] wrote that according to the opinion that it is a ספק חיה ספק בהמה we don't need a pasuk [as the Gemara says that we don't need a pasuk to forbid a safek because Hashem has no sfeikos with regard to whether it is a חיה or בהמה]. But that is all according to their opinion that "כל חלב" is referring to the opinion that a כוי is an independent species which is neither a חיה nor a בהמה and is thus not a ספק. However, according to the Rambam who holds that it is an independent species but the rabbis didn't decide on its status - we need כל חלב to tell us that it is an איסור ודאי [and we have to say that here we DO need a pasuk to tell us about a ספק because it is a new independent species and not merely a ספק חיה ספק בהמה about which there is no ambiguity up in Shomayim].  

עפ"י ספר טעם ודעת