Bechorot 12: What Can/not be Exchanged for a Donkey for a Priest?

Shemot (13:13) has taught us tat a first born donkey is exchanged for a lamb that is then given to a priest.  Today's Mishna teaches that a lamb or a kid are appropriate for this exchange.  However, a car or an undomesticated animal cannot be used.  As well, a hybrid (cross between two species regarding chromosomes) or a koy cannot be used for this exchange.

The rabbis discuss the fact that sometimes different animals mate successfully and sometimes they miscarry or their babies are born stillborn.  The koy, a chayya or hehema, or a unique creature has not definite English translations.  Especially in Massekhet Bikkurim, the rabbis use the koy as an example a cross between a wild animal and a domesticated animal.  Thus it could be a test case for many different halachot.  Steinsaltz teaches that the debate around its identification began around the time of the Mishna.  Some Sages believed that a koy was the cross between a deer or similar animal with a goat while others argued that a koy was a unique animal, an Ayal ha-bar.