Chullin 80 - The Product of a Deer and a Goat
When one slaughters a wild animal, one has to cover its blood with earth. A koy, being partly a wild animal, may require covering its blood - if we attach any importance to the father - or it may not. Accordingly, the Sages do not allow to slaughter a koy on a holiday, because we may dig the earth only for clear cases of blood covering, and not for doubtful ones.
Some say that crossbreeding between a deer and a goat is impossible, and that the "koy" is perhaps a wild ram, or a separate species, not clearly wild or domestic.
Art: Roelandt Jacobsz Savery : A stag, deers, herons, goats, parrots and other animals in a forest
